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)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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