Merge main (anchored-turn chat scrolling) into perf/switch-and-scroll

Main replaced the chat timeline scroll engine while this branch was in
flight, which obsoletes two of its subareas and reshapes a third:

- Chat timeline: main's LegendList-based MessageList/ChatContainer win;
  the activation-overscan staircase targeted the removed tanstack path
  (LegendList provides adaptive rendering natively) and is dropped along
  with its test.
- Scroll shadows: main's hook-based masks stay (the virtualized list owns
  its scroll element — there is no wrapper to hand the styling to); the
  viewport-wrapper ScrollShadow rewrite, its index.css replacement, its
  test, and the call-site viewportClassName adaptations are reverted to
  main. The chat OverlayScrollbar keeps this branch's disableHorizontal.
- OverlayScrollbar: the direct-DOM rewrite lands, but binding now follows
  the live container node instead of binding once per ref object — the
  chat scroller remounts on every session switch, and a bind-once
  contract left the scrollbar attached to a dead element.
- Markdown renderer: the detached-DOM cache and warm-block fast path
  merge with main's block-commit reveal (enter cascade), streaming code
  highlighting, and gutter reservation; the per-block reconcile keeps
  both the decoration-refresh path and the reveal cascade.

bun.lock

@@ -167,6 +167,7 @@
         "@dnd-kit/core": "^6.3.1",
         "@dnd-kit/sortable": "^10.0.0",
         "@dnd-kit/utilities": "^3.2.2",
+        "@legendapp/list": "3.3.8",
         "@lezer/highlight": "^1.2.3",
         "@opencode-ai/sdk": "1.18.21",
         "@pierre/diffs": "1.3.0-beta.6",
@@ -918,6 +919,8 @@
 
     "@kwsites/promise-deferred": ["@kwsites/promise-deferred@1.1.1", "", {}, "sha512-GaHYm+c0O9MjZRu0ongGBRbinu8gVAMd2UZjji6jVmqKtZluZnptXGWhz1E8j8D2HJ3f/yMxKAUC0b+57wncIw=="],
 
+    "@legendapp/list": ["@legendapp/list@3.3.8", "", { "dependencies": { "use-sync-external-store": "^1.5.0" }, "peerDependencies": { "react": "*", "react-dom": "*", "react-native": "*" }, "optionalPeers": ["react-dom", "react-native"] }, "sha512-GM4Hca/6WDvcY33XXCieR9MaG9CoZmACzwqQwRhKFSaNKfQV1lTLiTOpWuA9wnE8n8+6WeA52DwNKC9yrnPPeg=="],
+
     "@levischuck/tiny-cbor": ["@levischuck/tiny-cbor@0.2.11", "", {}, "sha512-llBRm4dT4Z89aRsm6u2oEZ8tfwL/2l6BwpZ7JcyieouniDECM5AqNgr/y08zalEIvW3RSK4upYyybDcmjXqAow=="],
 
     "@lezer/common": ["@lezer/common@1.5.1", "", {}, "sha512-6YRVG9vBkaY7p1IVxL4s44n5nUnaNnGM2/AckNgYOnxTG2kWh1vR8BMxPseWPjRNpb5VtXnMpeYAEAADoRV1Iw=="],