refactor(cache): extract filesystem fingerprinting into vt_fs_fingerprint crate

[wan9chi]

Motivation

The logic that turns traced filesystem accesses into cache decisions — fingerprinting inputs, collecting outputs, and the read-write-overlap verdict ("did the task modify its own input?") — lived inside the execution engine, interleaved with process spawning, IPC, SQLite storage, archiving, and reporting. The overlap policy is unsettled and will be rewritten several times; until now the only way to exercise any of it was to run a whole task through the engine.

This extracts the filesystem story into a new crates/vt_fs_fingerprint crate where it can be driven directly with synthetic traces and temp directories:

• TaskFs::pre_run — before the task executes: validate the io configuration, snapshot the listed inputs, and (given a previous run's fingerprints) report the first input that changed.
• TaskFs::post_run — after it finished: judge the traced accesses and return either Conclusion::InputModified (caching unsound) or Conclusion::Cacheable with the run's opaque InputFingerprints and its outputs.

The engine keeps storage, archiving, env-fingerprint tracking, and spawning; its cache module shrinks to storage primitives and the lookup orchestration moves next to execution (fetch-first: the entry is fetched, then pre_run compares against it).

Behavior is preserved — the full e2e snapshot suite passes untouched. Two deliberate exceptions: which error is reported when several things fail at once may differ in edge cases, and the cache entry layout is restructured around the opaque fingerprint record, so CACHE_SCHEMA_VERSION bumps 18 → 19 (existing caches are ignored once via the versioned cache directory).

🤖 Generated with Claude Code

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[github-actions]

fspy benchmark

linux

dynamic/launch             change  +1.01%  [ -2.87% ..  +5.36%]  overhead   +64.07%
dynamic/access             change  +0.08%  [ -1.11% ..  +1.23%]  overhead   +48.11%
static/launch              change  +0.57%  [ -4.20% ..  +5.37%]  overhead  +165.63%
static/access              change  -0.40%  [ -4.30% ..  +1.12%]  overhead  +839.51%

macos

dynamic/launch             change  -0.22%  [ -3.77% ..  +4.33%]  overhead  +218.02%
dynamic/access             change  +0.28%  [ -6.86% ..  +4.88%]  overhead   +21.68%

windows

dynamic/launch             change  -1.55%  [ -6.00% ..  +3.19%]  overhead   +29.17%
dynamic/access             change  +0.18%  [ -1.40% ..  +1.04%]  overhead    +8.57%