contrib: add deterministic fuzz coverage mode #35809

Adds an optional integer coverage_check argument to deterministic-fuzz-coverage:

• 0: run both checks, preserving the current default behavior
• 1: only check each corpus input individually
• 2: only check all corpus inputs in one process

This makes it easier to isolate per-input nondeterminism from cross-input state leakage.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35809.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #35608 (contrib: Skip llvm-cov rendering for deterministic fuzz inputs by HowHsu)

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Hi @maflcko , any chacne to have a look at this one when you're available. Thanks.

tACK e01ca9bacdeb860291b1824cac7fb099a3b41111

Built the tool and ran all three modes against an instrumented binary I put together here, all green, and the check catches nondeterminism on both paths. Mode 0 came out identical to master.

Silly nit: could the motivation go into the commit message?