Roadmap Phase 3-8: unify config resolution and cache per-call waste

[sorafujitani]

Phase 8 of the ruby-prism migration plan (#110 Phase 3): one config system, and the per-call waste that the fork model used to hide is gone.

What changes for users

--config PATH actually works now. It was silently ignored by the formatter: the Ruby side only used it for file selection while Rust independently re-discovered config on every call. Now:

• rfmt --config custom.yml --write file.rb applies the file's formatting settings (verified end-to-end with a distinctive indent_width)
• an explicitly passed config that is missing or invalid fails loudly with exit 1, once — before, a Thor bug (exit_on_failure? undefined) meant even error paths exited 0
• rfmt config now prints the effective settings the Rust formatter actually resolved (via a new resolved_config_yaml FFI), instead of a Ruby-side approximation
• discovery (.rfmt.yml etc.) is unchanged for well-formed files; a broken discovered file now warns and falls back to defaults instead of failing every format (deliberate: an LSP mid-edit of .rfmt.yml should not make formatting impossible). Explicit-path errors stay loud — the asymmetry is documented and tested

Long-lived processes pick up config edits: discovery results are cached keyed by cwd + found-file mtime; editing .rfmt.yml between two in-process formats changes the output (tested). Known limit (in code): a config newly created in a parent directory is picked up on the next cache invalidation, not instantly.

Performance

Config::discover() was a cwd→root→home filesystem walk on every Rfmt.format call, and Formatter::new rebuilt the 23-rule registry each time. With the discovery cache (walk → one stat batch), an explicit-path cache, and a OnceLock registry:

	ms/file (in-process, lib corpus)
before this PR	0.357
after	0.190

Cumulative since the migration started: 4.28 → 0.19 ms/file (~22x).

Design notes

• FFI: two fixed-arity functions (format_code/1, format_code_with_config/2) instead of magnus variadic parsing; config crosses the boundary as a PATH so YAML parsing stays in one place
• Rfmt.format(source, config_path: nil) keeps the positional call compatible
• Configuration (Ruby) shrinks to file-selection/cache concerns; the three discovery implementations now share one search order (full dedup left out of scope, noted in the plan)

Verification

• rspec 166/0 (new: explicit-path honored / missing / invalid, mtime invalidation, CLI e2e); corpus check 46/46
• cargo test 145 (new cache tests: mtime reload, file-vanished re-walk, not-found caching with cwd-creation detection, explicit-path error semantics); clippy/fmt/rubocop clean
• Real CLI transcript in the PR discussion above; re-verified independently (indent honored, exit 1 on missing config)
• Benchmark: three runs, 0.190–0.207 ms/file