`raster_layer_cache` (one ~w·h·16-byte Rgba16Float CanvasPair per keyframe) had no
size cap — scrubbing a long timeline grew VRAM without bound (~33 MB/frame at 1080p),
the largest unbounded consumer. Added a recency LRU (RASTER_LAYER_CACHE_MAX = 12):
`ensure_layer_texture` bumps the frame to most-recent and evicts the oldest past the
budget; the shown frame (and any rendered this pass) is always most-recent so it's
never the victim. Evicted textures re-upload cheaply from the resident/faulted-in
pixels on revisit. `remove_layer_texture` keeps the LRU in sync.
F3 debug overlay now reports the tracked VRAM (raster cache MB + frame count), pushed
from the GPU brush whenever the cache changes (wgpu exposes no allocator query).
Skyler Lehmkuhl