Improve config resolution caching with directory-based cache

[Dunqing]

This PR improves the performance of config resolution by caching results at the directory level instead of per-file. When a config is resolved for a file, we cache the result for all parent directories up to (and including) the config's directory.

Why the previous caching wasn't optimal

The previous implementation cached config lookups by file path:

prettierConfigCache.remember(filePath, async () => {
  return await prettier.resolveConfigFile(filePath)
})

This had two problems:

1. Cache per file, not per directory — Every file got its own cache entry, even though all files in the same directory share the same config. Processing 100 files in /src/components/ created 100 cache entries all pointing to the same config.
2. No parent directory caching — When resolving config for /repo/packages/ui/src/Button.tsx, only that exact path was cached. A subsequent file at /repo/packages/ui/src/Input.tsx would miss the cache (different path) and re-resolve, even though the answer is identical.

The new approach

Cache by directory and populate all parent directories up to the config location:

For file at '/repo/packages/ui/src/Button.tsx' with config at '/repo/.prettierrc'

Caches:

• /repo/packages/ui/src -> /repo/.prettierrc
• /repo/packages/ui -> /repo/.prettierrc
• /repo/packages -> /repo/.prettierrc
• /repo -> /repo/.prettierrc

Now:

• All files in /repo/packages/ui/src/ share one cache entry
• Files in /repo/packages/ui/utils/ hit the cache at /repo/packages/ui
• Files in /repo/other/deep/path/ hit the cache at /repo

Benchmark

Benchmark environment

• CPU: Apple M4 Max (16 cores)
• Memory: 64 GB
• OS: macOS 26.1
• Node: v24.10.0

Benchmark on dub codebase (~3000 files, deep nesting):

Version	Time (mean ± σ)
Before	15.547s ± 0.581s
After	12.499s ± 0.688s

Result: 1.24× faster (~24% improvement)

Benchmark on headlessui codebase (~600 files, flatter structure):

Version	Time (mean ± σ)
Before	4.140s ± 0.044s
After	3.879s ± 0.031s

Result: 1.07× faster (~7% improvement)

The improvement scales with directory depth and file count.

Note: This PR was completed with the assistance of Claude Code