M4.5: RAM preview: render options in the frame key, least-recently-shown eviction, stale frames freed at once

- `FrameKey` carries a hash of the render options (`frames::opts_hash`): switching Fast
  Previews (Draft / Fast Draft) or Realtime Shadows no longer shows frames rendered the other
  way.
- Eviction drops the least recently shown frame (`Frames::get` touches it), not the oldest
  inserted: a looping preview a little over budget no longer evicts the frame it shows next.
- Frames of earlier project revisions are dropped on every change (`Frames::drop_stale`); they
  could never be shown again but held memory (and GPU textures) until evicted.
- The green cache bars (Timeline, Preview panel) count the frames of exactly what the viewer
  shows — scale, 3D view / region of interest and options (`EffectcraftApp::shown_series`) —
  instead of any frame at that scale.
- Playback loops compute the key's fixed part once (`EffectcraftApp::frame_series`).
- Docs: architecture.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/architecture.md

@@ -129,6 +129,13 @@ its evaluated inputs, excluding the transform. Static and transform-only layers
 editing one layer re-renders only that layer. Effects that read the clock directly are declared in
 `effects::TIME_DEPENDENT`, and a test checks every registered effect against that list.
 
+The viewer's **RAM preview** (`ui-egui/src/frames.rs`) keeps finished frames keyed by project
+revision, comp, frame, scale, 3D view / region of interest and a hash of the other render
+options (Draft / Fast Previews, shadows, nested switches), so frames rendered under other
+options are never shown. When the budget (Settings ▸ Memory & CPU) is full the least recently
+shown frames go first, and frames of an older revision are dropped as soon as the project
+changes. The timeline's green bar counts the frames of exactly what the viewer shows.
+
 A persistent **disk cache** (`render::disk_cache`, Settings ▸ Media & Disk Cache) backs both
 the layer cache and the viewer's RAM preview: layer buffers that were slow to render and every
 preview frame are written (LZ4, atomic rename, checksummed) under 128-bit content keys — the