fix(core): avoid eager directory snapshots

[kitlangton]

What

Avoid repeatedly rebuilding the filesystem search directory snapshot while ripgrep indexes a repository. Large repositories now pay the directory materialization cost only when a directory or mixed file search is requested.

Before / After

Before: Every indexed file updated the directory Set, then copied the entire set into state.directories. As the index grew, per-entry copies accumulated and could produce quadratic allocation and copying work before a user searched.

After: Indexing keeps files in an array and unique directories in the existing Set. File searches read the file array directly; directory and mixed searches materialize the current set at query time, preserving partial results while indexing continues.

How

• packages/core/src/filesystem/search.ts removes the eagerly synchronized directory array and reads the live directory set from find.

Scope

This does not change fuzzy ranking, ripgrep traversal, path formatting, result limits, the optional fff search backend, or existing tests.

Testing

• cd packages/core && bun typecheck
• Push hook: bun turbo typecheck --concurrency=3