docs: Phase 4 Tier 2 (decoded ImageCache LRU) done; note Tier 1 + prefetch

STREAMING_TO_DISK_PLAN.md

@@ -461,6 +461,19 @@ it resident. The heavy, evictable thing is the **image assets** referenced by fi
 from blob row or external `path`); Tier 2 = decoded pixels (`ImageCache` + GPU textures —
 the heavy one).
 
+**Progress (2026-06-21):**
+- **[DONE] Tier 2 — bound the decoded `ImageCache`.** 256 MB **usage-LRU**: every
+  `get_or_decode`/`_cpu` bumps the asset's recency; inserts past budget evict the least-recently-used
+  (a miss re-decodes from `asset.data`). Achieves usage-based eviction via render-access recency
+  (simpler than the frame→asset enumeration below; that enumeration is only needed for *prefetch*).
+- **[TODO] Tier 1 — lazy/droppable compressed bytes.** Currently `asset.data` is **eager-read** from
+  the container on load (Phase 3.5b). Make it lazy: give `ImageCache` the container path, load bytes
+  on a decode miss via `read_packed_media_readonly(path, asset_id)` (don't eager-read on open → instant
+  load; don't accumulate compressed bytes). Wiring wrinkle: the `ImageCache` lives in the stage, not the
+  App that holds the container path → thread the path through `SharedPaneState`.
+- **[TODO, optional] Prefetch via frame→asset enumeration** (4a below) — only if Tier-2 misses during
+  scrub/playback prove visible; the access-recency LRU already keeps rendered images resident.
+
 ### 4a. Frame→asset enumeration (incl. nested clips — see note below)
 A function `assets_needed_at(time) -> HashSet<Uuid>`: walk each visible vector layer's active
 `ShapeKeyframe`, collect `fill.image_fill` across its `VectorGraph.fills`, **recursing into