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/BUGS.md

@@ -100,6 +100,7 @@ a live account since.
 
 | Entry | Title |
 |---|---|
+| [B204](#b204--a-write-over-a-partial-edit-whose-base-was-never-read-fails-with-eio) | A write over a partial edit whose base was never read fails with `EIO` |
 | [B203](#b203--pdfs-unlock-leaves-a-running-daemon-a-spent-refresh-token) | `pdfs unlock` leaves a running daemon a spent refresh token |
 | [B202](#b202--a-refused-session-shows-as-offline-forever-and-nothing-says-to-sign-in-again) | A refused session shows as offline forever, and nothing says to sign in again |
 | [B201](#b201--after-signing-in-again-the-old-daemon-puts-its-spent-tokens-back) | After signing in again, the old daemon puts its spent tokens back |
@@ -306,6 +307,33 @@ a live account since.
 
 Newest first.
 
+### B204 — A write over a partial edit whose base was never read fails with `EIO`
+
+**Status:** Fixed, not verified live
+**Found:** 2026-10-09, by CI on 2a86830 (`one_client_on_a_good_link`, seed 1, which did not replay
+locally). A partial write to `x/f.txt`, a file renamed twice before, was renamed over `z/d`. A
+create over `z/d` a moment later failed with `EIO`, and the daemon logged "refusing write over
+incomplete queued revision".
+**Where:** `ProtonFs::serve_open` in `crates/pdfs-fuse/src/filesystem.rs`
+
+**Repro:** Write at an offset into a file this client never read, then open it for writing again
+before the drain uploads the first write.
+
+**Cause.** The release of a partial write fills its gaps from the content cache only. When the
+cache does not hold the base, the queued revision stays incomplete until the drain fills it from
+Drive, and a write open over an incomplete revision was refused. B123, B157, B169 and B170 each
+closed one way the cache could miss the base. Any other miss, such as a file never read here,
+still failed the next write open for the length of the drain debounce.
+
+**Fix.** A write open over an incomplete queued revision copies the blob and fills its gaps from
+the remote base with `Core::fill_gaps`, as the drain does before an upload. The open fails only
+when that fill fails, for example offline.
+
+**Test:** `a_partial_write_over_bytes_never_read_takes_another_write_at_once` in `sim/daemon.rs`.
+Without the fix the second write fails with `EIO`.
+
+---
+
 ### B203 — `pdfs unlock` leaves a running daemon a spent refresh token
 
 **Status:** Fixed, not verified live