fix(fuse): fill an incomplete queued revision's gaps on a write open

A partial write whose base was not in the content cache queued an
incomplete revision, and every write open over it was refused with EIO
until the drain uploaded it. CI's one-client-lan simulation hit this on
seed 1: a create over a file whose partial edit had not drained yet.

The write open now copies the blob and fills its gaps from the remote
base with Core::fill_gaps, as the drain does, and fails only when that
fill fails (B204).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EuyxByBpK3fSWtyPkugeHz

docs/CHANGELOG.md

@@ -12,6 +12,10 @@ scratch (user data in `staging/` and `recovery/` is never touched by this).
 ## [Unreleased]
 
 ### Fixed
+- **Writing to a file again soon after a partial write could fail with an I/O error.** When the
+  first write changed only part of a file this client had not read, a second write, or saving
+  over the file, failed until the first one reached Drive. It now fetches the rest of the file
+  and goes ahead.
 - **A CAPTCHA could not be solved on hardened systems.** When the verification page cannot open in
   the app, as on secureblue, the app now offers it in your browser and continues the sign-in
   once you are done. `pdfs login` now handles a CAPTCHA the same way instead of failing.