fix(fuse): keep the size of a file open for writing across re-listings

B109: an online create seals an empty file on Drive, and until the last
close the written bytes live only in the open handle. A re-listing of
the parent, a size upgrade or a create landing re-interned the node with
Drive's empty size, the kernel capped reads at that size, and SQLite
reported "database disk image is malformed". State::keep_open_write_size
now carries the handle's size and local mtime over in all three places.

The acceptance script waits for My files to mount after a daemon restart
instead of failing at once.

docs/CHANGELOG.md

@@ -40,6 +40,9 @@ scratch (user data in `staging/` and `recovery/` is never touched by this).
 - **Editors can save over a file several times in a row.** The fix for B104 could leave the
   upload of a replaced file queued, and the next save onto the same name failed with
   `Input/output error` (B106).
+- **Databases and other files kept open while written no longer read back empty.** When the
+  folder was listed again while a file was still open, its size dropped to what Drive held,
+  often 0 bytes, and SQLite reported `database disk image is malformed` (B109).
 - **Deleting files during a mirror pass no longer marks the folder as failed.** A file removed
   after the pass planned its upload was counted as an error, so the folder showed `error` until
   a later pass (B108).