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.

system/kernel/process.zig

@@ -192,9 +192,12 @@ fn system_call(state: *architecture.CpuState) void {
         .exit => {
             exit_code = architecture.systemCallArg(state, 0);
             // A scheduled process tears down fully (terminateCurrent); a borrowed
-            // test thread unwinds back to the kernel that entered it.
+            // test thread unwinds back to the kernel that entered it. A NONZERO
+            // code is a deliberate failure exit (`.aborted`): "the work exists
+            // but I could not do it" — supervisors restart those, unlike a clean
+            // `.exited` ("nothing for me here"), which they let lie.
             if (scheduler.currentIsUserProcess()) {
-                scheduler.current().exit_reason = .exited;
+                scheduler.current().exit_reason = if (exit_code == 0) .exited else .aborted;
                 terminateCurrent();
             } else architecture.userExit();
         },