fix(cache): Correct key collisions, anchor/whole-file isolation, and cache-hit detection for undefined values

[Copilot]

FileCache had three correctness bugs: |-separator key collisions, anchor-scoped parsed results polluting the whole-file cache slot, and false cache misses for legitimately-undefined parsed values (e.g. !ignore-rooted documents).

src/cache.ts

• Key collision: Replaced flat Map<string, CacheEntry> keyed by "filePath|anchor" with a nested Map<string, Map<string|undefined, unknown>>. Paths or anchors containing | can no longer collide.
• Anchor/whole-file isolation: Raw file content is now stored in a separate fileContents: Map<string, string>. Calling set(path, content, anchor, fileContent) no longer synthesises a whole-file parsed entry — a subsequent get(path) (no anchor) correctly yields a miss and triggers a fresh full-document parse.
• has() / hasAsync(): New methods distinguish a stored undefined value from a genuine cache miss, since get() returns undefined in both cases.

src/parser.ts

• Cache-hit detection now uses cache.has(...) instead of cached !== undefined, so !ignore-rooted files (which parse to undefined) are not endlessly reparsed.
• cachedContent guard changed from if (cachedContent) → if (cachedContent !== undefined) in both sync and async paths, so empty files cached as "" are not re-read from disk.

test/cache.test.ts

• Updated assertions for set(path, content, anchor, fileContent) — one parsed entry (anchor-scoped), zero whole-file parsed entry, getFileContent still returns the raw string.
• Added has() assertion to the "cache undefined values" case to cover the new API.

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