perf(worktrees): bound the duplicate-id scan in reuseEqualCatalogRows (#14271)

* perf(worktrees): bound the duplicate-id scan in reuseEqualCatalogRows

Rows sharing an id are scanned linearly with a deep compare each, so a bucket
of k duplicates costs O(k^2). Both callers key on ids that are unique by
construction, so this is a bound on damage rather than a live fix.

Cap the scan instead of adding a second index: reuse is only an optimization,
so a missed match yields a new object identity, never a wrong row. A fingerprint
index would buy a little more reuse in a case nothing reaches, at the cost of a
second equality implementation that must stay in step with catalogValuesEqual
with no automated guard.

Worst-case duplicate bucket, no matches: k=1000 134ms -> 1.2ms, k=2000 541ms ->
2.2ms. Unique-id path unchanged (2000 rows: 0.86ms both).

* docs(worktrees): lead the duplicate-id cap comment with its reachability

A reader hitting MAX_DUPLICATE_ID_SCAN should learn first that no caller
produces duplicate ids today, so the cap reads as bounding future damage rather
than fixing something live.

src/renderer/src/store/slices/worktree-catalog-reconciliation.test.ts

@@ -71,4 +71,44 @@ describe('reuseEqualCatalogRows', () => {
     expect(reconciled[0]).toBe(current[1])
     expect(reconciled[1]).toBe(current[0])
   })
+
+  it('reuses a match inside the duplicate-id scan window', () => {
+    const current = [
+      { id: 'dup', marker: 'a' },
+      { id: 'dup', marker: 'b' },
+      { id: 'dup', marker: 'c' }
+    ]
+
+    const reconciled = reuseEqualCatalogRows(current, [{ id: 'dup', marker: 'c' }])
+
+    expect(reconciled[0]).toBe(current[2])
+  })
+
+  // Without the cap this walks the whole bucket, so a 64-row bucket costs 64
+  // deep compares per incoming row. Counting reads keeps the guard deterministic
+  // — a wall-clock assertion would be flaky on shared CI runners.
+  it('caps the deep compares for one id instead of scanning the whole bucket', () => {
+    const bucketSize = 64
+    const current = Array.from({ length: bucketSize }, (_, index) => ({
+      id: 'dup',
+      marker: `previous-${index}`
+    }))
+    let reads = 0
+    const incoming = [
+      {
+        id: 'dup',
+        get marker(): string {
+          reads++
+          return 'matches-nothing'
+        }
+      }
+    ]
+
+    const reconciled = reuseEqualCatalogRows(current, incoming)
+
+    // No match, so the incoming row is kept — a missed reuse costs identity, never correctness.
+    expect(reconciled[0]).toBe(incoming[0])
+    expect(reads).toBeLessThanOrEqual(8)
+    expect(reads).toBeLessThan(bucketSize)
+  })
 })

src/renderer/src/store/slices/worktree-catalog-reconciliation.ts

@@ -30,6 +30,14 @@ function catalogValuesEqual(left: unknown, right: unknown): boolean {
   return true
 }
 
+// NOTHING HITS THIS TODAY: both callers key on ids that are unique by
+// construction, so buckets stay at 1-3. It only bounds the damage if that ever
+// changes — a same-id bucket costs one deep compare per candidate, so an
+// unbounded one is O(k^2). A cap beats a second index that would have to stay in
+// step with catalogValuesEqual, and reuse is only an optimization: dropping a
+// match past the window costs object identity, never correctness.
+const MAX_DUPLICATE_ID_SCAN = 8
+
 export function reuseEqualCatalogRows<T extends CatalogRow>(
   current: readonly T[] | undefined,
   incoming: readonly T[]
@@ -48,10 +56,18 @@ export function reuseEqualCatalogRows<T extends CatalogRow>(
   }
   const reconciled = incoming.map((row) => {
     const candidates = currentById.get(row.id)
-    const previousIndex = candidates?.findIndex((candidate) => catalogValuesEqual(candidate, row))
-    return previousIndex !== undefined && previousIndex >= 0
-      ? candidates!.splice(previousIndex, 1)[0]
-      : row
+    if (!candidates) {
+      return row
+    }
+    const scanLimit = Math.min(candidates.length, MAX_DUPLICATE_ID_SCAN)
+    for (let index = 0; index < scanLimit; index++) {
+      const candidate = candidates[index]
+      if (candidate !== undefined && catalogValuesEqual(candidate, row)) {
+        candidates.splice(index, 1)
+        return candidate
+      }
+    }
+    return row
   })
   return current.length === reconciled.length &&
     current.every((row, index) => row === reconciled[index])