Find-in-Page: Cached Flat Buffer with Lazy Invalidation

5e0c6a7

This commit introduces CachedMatchFinder, a new class wrapping TextIterator that maintains a flattened text buffer of all visible page content and caches find-in-page results. Cache invalidation is gated on domTreeVersion and styleRecalcCount. The architecture shifts from per-query full DOM traversal with a windowed SearchBuffer to a single cached flat buffer with unbounded ICU search.

Before:                                After:
  findNext()                            findNext()
    └─► TextIterator (full DOM walk)      └─► CachedMatchFinder
          └─► SearchBuffer (windowed)           ├─[valid]─► cached matches
                └─► match range                 └─[stale]─► rebuild buffer + ICU search

  Cache validity: domTreeVersion == saved && styleRecalcCount == saved

Shadow DOM traversal behavior is controlled by find options: flat-tree mode can traverse across shadow boundaries, while DoNotTraverseFlatTree mode respects scope isolation.

Significance

Unlike SearchBuffer's windowed approach, CachedMatchFinder allocates a single flat buffer for the entire document — pathologically large pages could cause large allocations, and the offset-to-BoundaryPoint mapping introduces a new class of potential off-by-one errors.

Audit directions