fix(fuse): give uploads under a low cap the time their share needs

The upload deadline assumed at least 32 KiB/s. Under an upload cap shared
by every drain worker and sync task, a large file ran out of time on every
attempt, took the mount offline and restarted from byte 0. The deadline
now counts the upload's share of the cap as the slowest rate when that
share is lower (B231).

docs/BUGS.md

@@ -100,6 +100,7 @@ a live account since.
 
 | Entry | Title |
 |---|---|
+| [B231](#b231--a-large-upload-under-a-low-upload-limit-never-finishes) | A large upload under a low upload limit never finishes |
 | [B230](#b230--a-create-queued-offline-adopts-another-devices-file-of-the-same-name) | A create queued offline adopts another device's file of the same name |
 | [B229](#b229--a-queued-write-can-be-lost-to-a-power-cut-after-close-returned) | A queued write can be lost to a power cut after close returned |
 | [B228](#b228--a-gap-filled-write-whose-upload-answer-was-lost-becomes-a-conflict-copy-of-itself) | A gap-filled write whose upload answer was lost becomes a conflict copy of itself |
@@ -333,6 +334,27 @@ a live account since.
 
 Newest first.
 
+### B231 — A large upload under a low upload limit never finishes
+
+**Status:** Fixed, not verified live
+**Found:** 2026-10-16, architecture audit (`FINDINGS.md` 1.9).
+**Where:** `upload_deadline` in `crates/pdfs-fuse/src/link.rs`; `TransferRegistry::upload_cap` in
+`crates/pdfs-fuse/src/transfers.rs`; the three uploads in `crates/pdfs-fuse/src/drain.rs`
+
+**Repro:** Set the upload limit to 512 KiB/s or less, and queue several large files.
+
+**Cause.** An upload's deadline allowed for at least 32 KiB/s. The upload cap is one budget
+shared by every running upload: up to 16 drain workers and 8 mirror sync tasks. Under a low cap
+each upload got less than 32 KiB/s, so a large file ran out of time on every attempt. The
+timeout took the mount offline, and on reconnect the upload started again from its first byte.
+
+**Fix.** The deadline counts an upload's share of the cap as the slowest rate when that share is
+below 32 KiB/s. Without a cap, or with a generous one, the deadline is unchanged.
+
+**Test:** `link::tests::an_upload_under_a_low_cap_gets_the_time_its_share_of_the_cap_needs`.
+
+---
+
 ### B230 — A create queued offline adopts another device's file of the same name
 
 **Status:** Fixed, not verified live