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/changes/SessionLens.tsx

@@ -54,6 +54,20 @@ export function SessionLens({ ops, loadError, onPreview, selectedCallId }: Sessi
     </button>
   )
 
+  // Sizing a row's content constructs a Blob (a UTF-8 encode of the whole
+  // edit) — that was per row PER RENDER, re-encoding every field on every
+  // poll tick. ops keeps its identity between unchanged polls (see the
+  // tab's opsRef), so one pass per changed fold covers every render.
+  const opSizes = useMemo(() => {
+    const sizes = new Map<string, string>()
+    for (const op of ops) {
+      if (op.kind !== 'read' && !op.isError) {
+        sizes.set(op.callId, formatBytes(new Blob([op.edit?.newString ?? op.content ?? '']).size))
+      }
+    }
+    return sizes
+  }, [ops])
+
   return (
     <div className={css.session}>
       <div className={css.filterRow} role="group" aria-label={t('changesSessionLens')}>
@@ -85,9 +99,9 @@ export function SessionLens({ ops, loadError, onPreview, selectedCallId }: Sessi
                 {op.running && <span className={css.opFlag}>{t('changesRunning')}</span>}
                 {op.isError && <span className={css.opFlagError}>{t('changesError')}</span>}
                 <span className={css.opMeta}>
-                  {op.kind !== 'read' && !op.isError && (
+                  {opSizes.get(op.callId) !== undefined && (
                     <span className={css.opSize}>
-                      {formatBytes(new Blob([op.edit?.newString ?? op.content ?? '']).size)}
+                      {opSizes.get(op.callId)}
                     </span>
                   )}
                   <span className={css.opTime}>{relativeTime(new Date(op.time).toISOString())}</span>