volume-manager: rebuild a volume when its storage driver dies (S5)

The V4 review's open edge: a storage driver that crashes while its device
stays in the tree left fat wedged on a dead channel — device-presence
polling (a device-manager enumerate) still reported the device present,
so nothing reaped it. pollTick now also probes channelAlive(dev), a
geometry() on the block channel that fails fast on the dead endpoint; a
present device with a dead channel is reaped like a pull, and the adopt
loop re-adopts it on the restarted driver's fresh channel — the rebuild.
The manager's own liveness probe makes fat self-detection unnecessary:
it rebuilds regardless of the wedged filesystem's state.

The drill: the device manager gains a test-storage-restart mode that
kills usb-storage once, ~2s after its hello (post-mount); a new
volume-driver-restart kernel case boots a manual tree with it, and the
QEMU case asserts a SECOND mount of the same id-path after the reap —
the rebuild. A pre-S5 manager, checking only device presence, never
reaps, so the second mount never appears. The manually-spawned volume
manager needed kernel-supervisor protocol grants (bind its name, open
the device manager), as the other manual-tree services already have.

Full suite 133/133.

test/qemu_test.py

@@ -815,6 +815,23 @@ CASES = [
                r"[\s\S]*volume-manager: medium left a storage device; unmounting"
                r"[\s\S]*volume-manager: storage for volume \d+ removed; unmounting",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},
+    # S5 storage-driver-crash rebuild. The device manager (test-storage-restart
+    # mode) kills usb-storage once, ~2s in — after its volume mounted. The device
+    # stays in the tree, so device-presence polling alone would leave fat wedged on
+    # the dead channel; the volume manager's channel-liveness probe (a geometry()
+    # that fails on the dead endpoint) must notice, reap the volume, and rebuild on
+    # the restarted driver's fresh channel — a SECOND mount of the same id-path.
+    # Discrimination: a pre-S5 manager checks only isDevicePresent (still true), so
+    # it never reaps and the second mount never appears (it would restart fat on
+    # the stale channel and crash-loop).
+    {"name": "volume-driver-restart",
+     "build_case": "volume-driver-restart",
+     "smp":    4,
+     "timeout": 150,
+     "expect": r"(?s)fat: mounted /volumes/fat-12345678"
+               r"[\s\S]*volume-manager: storage for volume \d+ removed; unmounting"
+               r"[\s\S]*fat: mounted /volumes/fat-12345678",
+     "fail":   r"failing repeatedly; giving up|\[FAIL\]|DANOS-TEST-RESULT: FAIL"},
     # Volume-manager discovery + probe (V3a, docs/volume-manager-plan.md). Reuses
     # the fat-mount kernel build (the default boot now spawns the volume manager
     # from init.csv). It acquires the mass-storage block channel through the