M4.14: content-keyed RAM preview frames; proxies in the disk cache's content key

- FrameKey's identity is the comp's content (frames::comp_content), not the project revision:
  an edit keeps the cached frames of comps it doesn't touch and undo finds the frames of the
  state it returns to. The identity hashes the addresses of the comps a comp draws (~0.1 ms on
  the 1500-layer test project; the full content hash took ~117 ms), by value the footage, solids,
  proxies and project settings; with expressions, every item. Frames::content_of keeps a
  project snapshot per identity with frames (or in flight), so Arc::make_mut always copies an
  edited comp and its address can't be reused meanwhile; past 64 identities the least recently
  used go with their frames (long drags).
- drop_stale is gone (frames of other contents are evicted least-recently-shown first).
- disk_cache::comp_content_key covers items' proxies and Use Proxy (it served stale frames).
- Tests: frames (content keys, edits elsewhere and undo, bounded drags, in-place edit safety),
  ui_ram_preview (end to end), the disk cache proxy key. Docs: roadmap (M4 done), parity.

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

crates/render/src/disk_cache.rs

@@ -778,8 +778,9 @@ pub fn footage_salt(project: &effectcraft_project::Project) -> u64 {
 }
 
 /// Content key of a composition's pixels: project render settings, the comp and everything it
-/// uses (nested comps, footage, solids, footage files' size and time). With expressions in play
-/// (which can read any comp) the whole project counts.
+/// uses (nested comps, footage, solids, their proxies and Use Proxy switches, footage and proxy
+/// files' size and time). With expressions in play (which can read any comp) the whole project
+/// counts.
 pub fn comp_content_key(project: &effectcraft_project::Project, comp: effectcraft_project::ItemId) -> u128 {
     use effectcraft_project::{ItemKind, LayerSource};
     let mut h = Hash128::default();
@@ -813,6 +814,10 @@ pub fn comp_content_key(project: &effectcraft_project::Project, comp: effectcraf
         if let ItemKind::Footage(f) = &it.kind {
             footage_stamp(&mut h, f);
         }
+        hash_debug(&mut h, &it.proxy);
+        if let Some(px) = &it.proxy {
+            footage_stamp(&mut h, &px.footage);
+        }
     }
     h.write_u64(comp.0);
     h.finish()