Fills the gap where the committed-ink Picture cache (the P0.5 perf primitive:
record once per revision, replay every frame) was only covered indirectly via
StaticInkPainter. Verifies same-(host,revision) builds exactly once then returns
the cached Picture; a new revision rebuilds; host ids are independent; LRU
eviction beyond maxSize keeps the most-recently-used and disposes the rest
post-frame.
flutter analyze lib/editor clean; 244/244 tests (+4).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
