`RasterStrokeAction`/`RasterFillAction` stored the whole before+after RGBA frame
(~16 MB/action at 1080p → up to ~1.6 GB at the 100-action cap). They now store a
`RasterDiff` — only the changed bounding box's pixels before and after — computed
once in `new()` from the full buffers, which are then dropped. A brush dab shrinks
from ~16 MB to tens of KB; a full-canvas fill is unchanged (its bbox is the frame).
Paging interaction: a diff overwrites just the bbox, so the keyframe's pixels must
be resident when undo/redo applies. A clean evicted frame's container bytes equal
its current logical state, so the editor faults the target frame in (synchronously)
before undo/redo via a new `Action::raster_resident_hint` + `peek_undo/redo_raster_hint`.
Dirty frames are never evicted, so they're already resident. If a base is somehow
not resident the apply is skipped (logged), never resized-and-corrupted.
Unit tests cover exact before/after round-trip, blank-first-stroke, no-op, and the
non-resident-base skip.
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