Predicate Caching Impl

[kurasaiteja]

What does this PR do?

Implements the predicate-caching RFD (#262). Custom predicates now emitWatch{File,Env,Time} JSONL events on stdout. Symposium caches their exit-coderesult in ~/.symposium/cache/predicates/<workspace-hash>.json and skips
re-evaluation until a watched file, env var, or TTL becomes stale.

Three granular events on a #[non_exhaustive] CustomPredicateEvent enum

• WatchFile(PathBuf) result depends on this file's mtime + size.
• WatchEnv(String) result depends on this env var's current value.
• WatchTime(u64) result becomes stale after this many milliseconds.
• WatchTime(0) disables caching.

Exit status still determines pass/fail. Events only control cache lifetime.

Cache layout

Per-workspace cache file at <SymposiumDirs::cache_dir>/predicates/<sha256(canonical workspace_root)>.json
I think isolating by workspace prevents two projects on the same machine that emit the same predicate string from stepping on each other's cached answers. Schema version tag invalidates on Symposium upgrade.

Note

• Built-ins (path_exists, env, shell) stay uncached for now. path_exists is one stat, env is one hashmap lookup and the cache check would cost the same as just running them, so caching them doesn't buy anything. If it's useful to route them through the cache for observability, happy to add.

Disclosure questions

AI disclosure.

• I used an AI tool for research, autocomplete, or in other minimal ways
• The AI tool authored small parts of the code (e.g., autocomplete, comments)
• The AI tool authored large parts of the code
• Other: (explain)

Questions for reviewers.