fix(fuse): a file landing while its folder is pinned stays available offline (B208)

A folder pin reads each file's modification time and size before it
downloads the file. If the file's create landed during the download, the
landing set Drive's time, and the pin then stored the blob under the old
time. The file showed as kept offline but read as stale.

After each download, the pin now compares the file's time and size with
the tree's. If they changed, it downloads the file again under the new
values, at most three times.

The simulation gains FakeClient::hold_next_download to force that order.

docs/BUGS.md

@@ -100,6 +100,7 @@ a live account since.
 
 | Entry | Title |
 |---|---|
+| [B208](#b208--a-file-landing-while-its-folder-is-pinned-is-kept-but-not-available-offline) | A file landing while its folder is pinned is kept but not available offline |
 | [B207](#b207--a-move-between-locations-goes-ahead-under-a-file-opened-just-after-its-create-landed) | A move between locations goes ahead under a file opened just after its create landed |
 | [B206](#b206--an-upgrade-leaves-the-old-daemon-running-and-nothing-says-so) | An upgrade leaves the old daemon running, and nothing says so |
 | [B205](#b205--a-photo-upload-the-sdk-refuses-says-youre-offline) | A photo upload the SDK refuses says "You're offline" |
@@ -310,6 +311,34 @@ a live account since.
 
 Newest first.
 
+### B208 — A file landing while its folder is pinned is kept but not available offline
+
+**Status:** Fixed, not verified live
+**Found:** 2026-10-11, CI simulation run of `3e4886f`.
+`sim::daemon::tests::a_pinned_folder_keeps_every_file_under_it_until_it_is_unpinned` waited
+30 s for `Docs/sub/b.txt` to read as cached and gave up. It failed about once in 25 runs under load
+and never alone.
+**Where:** `Core::pin_subtree` in `crates/pdfs-fuse/src/lib.rs`
+
+**Repro:** Pin a folder while a create in it lands, after the drain has repointed the tree at
+the real uid and before it has read the node back. The file's cached blob must be gone, as after
+`pdfs cache clear`, so the pin downloads it.
+
+**Cause.** The pin walks the folder first and downloads each file under the modification time
+and size it read then. The landing's read-back then sets the time Drive gave the file, and keeps
+the uploaded bytes under it. The pin's download ended after that and stored the blob again under
+the old time. The cache records one time per blob, so the file read as stale: shown as kept
+offline, but downloaded again on the next read, and unreadable offline.
+
+**Fix.** After each download the pin compares the file's time and size with the tree's. If they
+moved, it fetches the file again under the new ones, at most three times.
+
+**Test:** `sim::daemon::tests::a_folder_pinned_while_a_create_in_it_lands_keeps_that_file` holds
+the read-back and the pin's download with `FakeClient::hold_next_download`, new for this, and
+lets them go in that order.
+
+---
+
 ### B207 — A move between locations goes ahead under a file opened just after its create landed
 
 **Status:** Fixed, not verified live