fix(files): stop file viewer reload loop from sub-ms mtime jitter (#1489) (#2297)

* fix(files): guard file polling races

- Ignore sub-millisecond mtime jitter on a same-size file so an unchanged
  open file no longer loops through reload and flickers.
- Swap externally changed text content into the open editor in place
  instead of clearing the loaded path and showing the load spinner.
- Read content only after metadata changed, confirm it with a second
  read, and skip the swap when the file is unchanged, the buffer is
  dirty, or a newer local write landed.
- Keep the stat baseline unchanged when a poll cannot observe content so
  a failed read is retried rather than treated as unchanged.
- Fall back to a full reload for images, PDFs, binaries, and files above
  the content-poll byte limit, and when a poll returns binary content.
- Serialize polls and dispose the poller on unmount, file switch, and
  directory change.
- Add a `fresh` file read option that bypasses the content cache and the
  HTTP cache.

* fix(files): invalidate stale polls after diagram saves

---------

Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>

packages/ui/src/components/views/fileStatChange.ts

@@ -0,0 +1,16 @@
+type FileStatChangeInput = {
+  size: number;
+  mtimeMs?: number;
+};
+
+// Some filesystems report sub-millisecond mtime jitter for unchanged files.
+const MIN_MTIME_CHANGE_MS = 1;
+
+export const hasFileStatChanged = (
+  previous: FileStatChangeInput,
+  latest: FileStatChangeInput,
+): boolean => latest.size !== previous.size || (
+  latest.mtimeMs !== undefined
+  && previous.mtimeMs !== undefined
+  && Math.abs(latest.mtimeMs - previous.mtimeMs) >= MIN_MTIME_CHANGE_MS
+);