fix: tighten as any usage in performanceShim and clarify fileHistory cap

- performanceShim: replace 'as any' on the onresourcetimingbufferfull
  getter/setter and the globalThis install guard with typed intersection
  casts, per the project's TS guideline.
- fileHistory: clarify the MAX_SNAPSHOTS comment so it reflects that
  checkpointing is throttled (cap reduced from 100 to 20), not disabled.

src/utils/fileHistory.ts

@@ -51,8 +51,9 @@ export type FileHistoryState = {
   snapshotSequence: number
 }
 
-// Disabled: file checkpointing causes unbounded memory growth (100 snapshots × full file backups).
-// See heap snapshot analysis — re-enable only after switching to incremental diffs.
+// Throttled: file checkpointing remains enabled (gated by fileHistoryEnabled())
+// but capped to mitigate unbounded memory growth (full file backups x N snapshots).
+// See heap snapshot analysis; only raise this cap after switching to incremental diffs.
 const MAX_SNAPSHOTS = 20
 export type DiffStats =
   | {

src/utils/performanceShim.ts

@@ -145,10 +145,11 @@ const shim = {
     return original.timeOrigin
   },
   get onresourcetimingbufferfull() {
-    return (original as any).onresourcetimingbufferfull
+    return (original as Performance & { onresourcetimingbufferfull?: unknown })
+      .onresourcetimingbufferfull
   },
-  set onresourcetimingbufferfull(_v: any) {
-    // no-op — prevent accumulation
+  set onresourcetimingbufferfull(_v: unknown) {
+    // no-op to prevent accumulation
   },
   toJSON() {
     return original.toJSON()
@@ -161,8 +162,11 @@ const shim = {
  * native Performance reference.
  */
 export function installPerformanceShim(): void {
-  if ((globalThis as any).__performanceShimInstalled) return
-  ;(globalThis as any).__performanceShimInstalled = true
+  const g = globalThis as typeof globalThis & {
+    __performanceShimInstalled?: boolean
+  }
+  if (g.__performanceShimInstalled) return
+  g.__performanceShimInstalled = true
   globalThis.performance = shim
 }