usb/fat: transfer events matched by slot+endpoint; storage failures heal

B2 — the 1-in-3 boot-time READ CAPACITY failure, root-caused: the xHCI
library's awaitTransfer claimed ANY unclaimed transfer event as its own
completion. An interrupt-endpoint event whose TRB pointer no longer
matched the armed subscription (an error or stale completion from the
keyboard/mouse polling concurrently with storage bring-up) fell through
and was misread as the bulk transfer's completion — desynchronizing the
mass-storage bulk protocol in controller state that SURVIVED driver
restarts, so every retry failed too. Awaited transfers now match the
event's slot id and endpoint DCI; foreign events are dropped and named.
Twelve consecutive runs of the previously-flaky cases pass; the full
suite is green with none of its old intermittents.

B1 — and when storage does fail transiently, the system now heals
instead of giving up forever: a nonzero exit maps to ExitReason.aborted
(a deliberate FAILURE exit — supervisors restart those with backoff,
unlike a clean .exited), usb-storage exits nonzero when a PRESENT
device fails bring-up, and the fat service no longer blocks its harness
polling for a block device and then dies — it serves immediately
(requests fail politely), retries on a 500 ms timer, and mounts
whenever storage appears, including after a driver restart.

library/runtime/block.zig

@@ -54,6 +54,13 @@ pub const Device = struct {
     }
 };
 
+/// One lookup attempt, no waiting — for a server that retries on its own
+/// timer (the fat service) instead of blocking its harness in here.
+pub fn tryOpen() ?Device {
+    if (ipc.lookup(.block)) |handle| return .{ .endpoint = handle };
+    return null;
+}
+
 /// Look up the block device, retrying generously while the USB storage chain
 /// (controller reset, enumeration, mass-storage bring-up) comes up.
 pub fn open() ?Device {