[Demo] validation: remove AssumeUTXO #36598

Use `AddCoin` and `SpendCoin` to set up the flush, reset and overlay tests, leaving direct coverage of `EmplaceCoinInternalDANGER` in the simulation, fuzz target and duplicate-insertion test.
Set `possible_overwrite=true` to keep the entries DIRTY but not FRESH, as before.

Preserve tip notifications, candidate rebuilding and explicit rollback exports before removing mixed snapshot tests.

Keep the existing snapshot-first cache split after the snapshot reaches tip.
Remove the IBD-exit resize trigger and the allocation reversal that prioritizes historical validation.
Activation and startup still resize caches while snapshot loading is supported.
The existing allocation test no longer needs to force IBD to select this split.

Remove the backgroundvalidation object from getblockchaininfo and its dedicated assertions.
Snapshot import, background validation and the getchainstates diagnostic remain available.

Use the ordinary missing-block rescan errors instead of making wallets depend on background progress.
Keep the snapshot wallet-load restriction and describe explicit reindex recovery.
Update only the existing error assertions while background validation and wallet scenarios remain supported.

Use the active chain and existing block-availability checks for index startup.
Reject snapshot imports while indexes are running and remove automatic index restarts at snapshot completion.
Keep background validation and its network-service restoration.
The existing snapshot test starts indexes on its ordinary restart after historical validation completes.

Download blocks only for the active chain and stop historical activation at its common entry point.
Keep the guard with download removal so startup and direct callers cannot advance incomplete historical candidates.
Adapt the existing import, wallet and native scenarios to stopped history and explicit reindex recovery.

Historical activation is already disabled. Remove its callers in ProcessNewBlock and ActivateBestChains.
Remove completion handling from ConnectTip and the remaining network-service callback.
Keep periodic flushing and the genesis interrupt ordering.
Snapshot imports and startup resumption remain supported.

Remove historical UTXO hashing, its failure handling and the restart-time directory normalization it drove.
Remove the hash-mismatch test and reuse the completion scenario to check that restart keeps the snapshot assumed.
Snapshot loading still checks the compiled commitment before activation.
Snapshot databases now remain assumed across restart until explicitly discarded by reindexing.
Previously completed imports can be normalized with a compatible older release before upgrading.
Remove background progress and prefixes from the existing tip-log helper.

Retire loadtxoutset after background validation has been removed independently.
Keep a hidden RPC tombstone and string argument conversion to explain the removal to existing callers.
Existing snapshot datadirs still resume, so keep getchainstates and startup coverage until resumption is retired.
Remove import instructions and end-to-end import scenarios, preserving ordinary kernel-tool coverage.
Reuse the export test to check the removed RPC's error code with both RPC and CLI transports.

AssumeUTXO import and completion were the only runtime writers of local services.
Their removal lets CConnman store the startup services without an atomic.
Init sets them before network threads start and before RPC warmup ends.
Keep the constructor's initial NODE_NONE value through its existing Init call.

After snapshot import removal, refuse snapshot datadirs with an active marker before destructive node or kernel initialization.
Keep normalized ordinary datadirs usable and preserve rejected data for recovery with a compatible older release.
Remove getchainstates and activation, restart and import fuzz coverage now that supported entry points are closed.
Keep cache resizing, commitment and metadata parsing tests until their corresponding APIs are removed.
Document this transition alongside the compatibility tests.
Delete candidate clearing with its last snapshot-test caller.
Remove unused database and validation-cache fixture options with their last snapshot test callers.

Keep UTXO exports available without trusted snapshot commitments.
Require a rollback height or hash instead of choosing a supported AssumeUTXO snapshot.

Remove the trusted snapshot entries now that import and startup resumption are unavailable.
Keep the lookup interfaces until their remaining callers are removed.

Snapshot startup and imports are disabled. Remove the remaining snapshot-base peer filter and historical fork-warning exception.

Snapshot imports and startup resumption are unavailable.
Initialize and verify the ordinary chainstate directly, retaining interruption and database errors.
Completion hashing and directory normalization were removed with background validation.
Delete the unused restart loader and chainstate deletion helper with their last startup callers.
Remove the deletion test while retaining ordinary kernel operation without a mempool.

Delete snapshot activation, coin import and their unused helpers.
Keep AddChainstate until its remaining cache-rebalancing test is removed.

Snapshot import bypassed the normal AddCoin spentness checks through EmplaceCoinInternalDANGER.
With the importer removed and fixtures using AddCoin and SpendCoin, delete the bypass and its entry constructor together with the remaining simulation, fuzz and duplicate-insertion coverage.

AssumeUTXO activation and cache sharing no longer need runtime resizing.
Remove the final startup rebalance call and the unused rebalancing and resizing APIs.
Keep startup cache-budget storage and use the existing test budget field directly.
The database is no longer replaced, so compaction needs no extra mutex.
Remove AddChainstate with its final cache-rebalancing test caller.
Remove the resizing and rebalancing tests with the APIs they exercise.

Use the ordinary chainstate directory without snapshot path selection.
Remove the base-block metadata readers, writers and StoragePath helper.
Retain the UTXO export file format and its metadata parser.

Snapshot loading has already been removed, so candidates require transaction history back to genesis.
Remove snapshot-base exceptions from transaction counts and consistency checks.
Ordinary script, sigop, BIP30 and proof-of-work checks are unchanged.

Reconstruct cumulative transaction counts from downloaded block history.
Remove the trusted snapshot count supplied during block-index loading.
Remove the commitment type, storage and lookup interfaces with their last caller.
Delete FindFirst with the lookup methods that used it.
Include util/vector.h directly for the external signer now that chainparams no longer supplies it.

Without AssumeUTXO, historical and tip downloads no longer need separate block files or divided prune budgets.
Write blocks and undo data through the ordinary block-file cursor.
Keep cs_main locking, undo flush failures and pruning retention unchanged.
With one cursor, flushing always targets the current block file. Remove the separate file selector and forwarding wrapper.
The undivided prune target already satisfies the configured minimum, so it needs no per-chainstate clamp.
Delete the obsolete cursor selector and printer with their last callers.
Remove the snapshot lower prune bound while preserving the existing pruning helper interfaces.

Retain pruning and rescan errors without the removed AssumeUTXO state.
Do not change wallet transaction accounting.
Remove hasAssumedValidChain with its last wallet callers.

All remaining notifications come from the ordinary chainstate.
Remove the role argument from internal callbacks and their callers.
The public kernel callback ABI is unchanged.
Delete the unused role accessor, formatter and type with their consumers.

Use the active chainstate for indexing and progress.
No snapshot chainstate can be constructed or loaded.
Remove the unused snapshot target, base and chainstate-selection methods with their last callers.

Replace the chainstate vector with one owned chainstate.
Remove snapshot targets, validity state and obsolete aliases.
Snapshot activation no longer transfers the optional mempool between chainstates.
Keep its pointer mutable for fuzz fixtures that replace the pool.
Keep shutdown flushing and block activation on the ordinary path.
Remove ResetChainstates with its last caller instead of rewriting its unused body.
A successfully created kernel manager always owns that state at destruction.
Tip notifications no longer need to check which chainstate is active.

Deleting the AssumeUTXO importer leaves only the export side of the snapshot metadata format.
Delete the unused metadata parser, its deserialization fuzzer and the parser-only network-magic lookup.
Keep the existing export serializer and its callers unchanged.

AssumeUTXO completion no longer tears down and reloads chainstates during startup.
Fold the remaining initialization pass into LoadChainstate and pass the cache budgets directly to the sole chainstate instead of retaining totals for redistribution.

Removing AssumeUTXO cache resizing leaves the coins database open for the lifetime of its view.
Store the wrapper by value and retain only the path used in compaction logs instead of parameters for reopening it.
Pass database parameters by reference now that neither view retains them.
Keep the compaction future, overlap check and destructor wait so a worker cannot outlive the database.

Historical AssumeUTXO downloads were the only caller that omitted the active chain and staller tracking.
Fold the shared traversal into the ordinary download path and evaluate its in-flight budget once now that there is no second download pass.

The snapshot base was the only nonzero lower bound on pruning.
With AssumeUTXO removed, keep the tip-distance and requested-height limits in block storage and drop the always-zero lower bound and chainstate range accessor.

Snapshot activation no longer replaces the active chainstate while the node is running.
Read the fixed owner without cs_main and remove locks that existed only to select it, including the outer fee-estimation lock around a block assembler that locks internally.
Chain contents and validation still retain their existing synchronization.

With AssumeUTXO gone, activation visits one chainstate and no longer needs to identify which state failed.
Use the singular activation name and remove the chainstate formatter, its startup log and the lock used only to format the activation error.

Removing snapshot imports leaves dumptxoutset as the only consumer of the snapshot header and export helpers.
Write the unchanged version 2 header at its sole call site and keep the helpers inside the RPC translation unit.
Drop unused default arguments from these now-local export helpers.