M4.8: docs: parity covers the cached playback fixes

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/parity.md

@@ -383,6 +383,23 @@ Browser in the web app (landed in M13.10: browser storage and File System Access
   wrong signatures and versions, fuel exhaustion, traps and bit-identical output; script
   `Socket` validates ports, and ScriptUI resource strings accept trailing array commas.
 
+## Update: M4.5–M4.8 cached playback
+
+- **RAM preview keys** carry the render options (Fast Previews Draft / Fast Draft, Realtime
+  Shadows), so a mode switch never shows frames rendered the other way; eviction drops the least
+  recently shown frame; frames of an earlier revision are freed as soon as the project changes;
+  the green bars count exactly what the viewer shows (scale, 3D view, region of interest).
+- **Playback**: viewer Resolution Auto keeps its scale while playing (it halved it, so cached
+  frames weren't reused); Cache Before Playback cuts a range larger than the RAM preview budget
+  to the frames that fit and plays them (it waited forever); the Info panel shows the achieved
+  frame rate and whether it is real time.
+- **Cache Frames When Idle** renders the work area in the background after a second of quiet.
+- **Edit ▸ Purge**: disk and snapshot purges keep the RAM preview; memory purges also drop
+  decoded footage frames.
+
+Not yet: content-keyed RAM frames (an edit drops every comp's RAM frames; the disk cache keeps
+content keys), audio scrubbing, four snapshot slots.
+
 ## Update: M3.14 project files
 
 Project files name the version that wrote them (`savedBy`); opening one that a newer EffectCraft