perf: stabilize renders (transcript row reuse, tree Sets, shared + options, batched drags)

SideChat transcript: every 2s poll rebuilt ALL row objects, so every
row (markdown parse + DOMPurify included) re-rendered even when the
log had not changed. transcriptRows now takes the previous poll's
rows and re-adopts the position-aligned rows whose content is
identical (reuseRows) — only the changed tail rebuilds. The view
feeds the memoized array back through a ref; unit tests pin the
identity contract (unchanged rows keep object identity, changed rows
and everything past the change rebuild, tool-result landings never
re-adopt).

FileTree: expanded/revealed membership was an includes() scan per
rendered row (O(rows × expanded) per render on deep trees); one Set
per render keeps it O(1) per row.

Shell: buildNewTabOptions was inlined in the two Workbench JSX
sites — two fresh arrays per render (drag state, resizes) re-rendered
every LeafView's + affordance. One shared memo now.

DiffPane resize / TerminalView ResizeObserver: pointer and resize
streams fired a setState / glyph-measuring fit() per event; both ride
per-frame batching now (frame-batcher in the core, a local rAF gate
in the lazy terminal chunk). SessionLens sizes each row's content
with a Blob (a UTF-8 encode of the whole edit) — that moved from
per-row-per-render into one memoized pass per changed fold.

src/client/SideChatView.tsx

@@ -383,6 +383,8 @@ export function SideChatView(props: {
   const [menuOpen, setMenuOpen] = useState(false)
 
   const cacheRef = useRef<ThreadCache>({ entries: [] })
+  // The previous poll's rows (see the mapping's reuse pass below).
+  const prevRowsRef = useRef<SidechatTranscriptRow[]>([])
   const controllerRef = useRef<AbortController | null>(null)
   const scrollRef = useRef<HTMLDivElement | null>(null)
   const composerRef = useRef<HTMLTextAreaElement | null>(null)
@@ -480,6 +482,7 @@ export function SideChatView(props: {
   // the composer — it owns the first message of a fresh thread.
   useEffect(() => {
     cacheRef.current = { entries: [] }
+    prevRowsRef.current = []
     controllerRef.current?.abort()
     setError(null)
     setSaved(false)
@@ -517,8 +520,14 @@ export function SideChatView(props: {
     }
   }, [connectionState, threadId, fetchThread, fetchInfo])
 
-  const rows = useMemo(
-    () => (threadId === undefined ? [] : transcriptRows(cacheRef.current.entries)),
+  // The previous poll's rows ride into the mapping so unchanged rows keep
+  // their object identity (see reuseRows): the 2s poll re-renders only the
+  // changed tail instead of re-parsing markdown for the whole transcript.
+  const rows = useMemo(() => {
+    const next = threadId === undefined ? [] : transcriptRows(cacheRef.current.entries, prevRowsRef.current)
+    prevRowsRef.current = next
+    return next
+  },
     // The cache is a ref; revision bumps on every successful pull.
     // eslint-disable-next-line react-hooks/exhaustive-deps
     [threadId, revision],