volume-manager: discovery and probe — V3a

The storage layer gains its policy home (storage-architecture.md): a new
system/services/volume-manager, spawned by init, that acquires the mass-
storage block channel through the device manager (the same lineage a
filesystem uses), reads block 0, and parses the first volume out of it. The
partition-table walk that lived in the FAT engine moves here, above the
driver where it belongs (partition.zig, host-tested: MBR entry, bare-FAT,
no-signature). Identity is the MBR disk signature + partition index — the
weak rung of the ladder; GPT GUID and FAT serial refine identityOf without
changing shape.

This increment is discovery + probe + log only, additive: the FAT service
still acquires its own volume, so nothing changes for it. Confining each
filesystem to its partition and spawning one per volume (the flip) lands
next, keeping fat working throughout.

Grants + wiring: init.csv spawns it after the device manager; protocol.csv
grants bind volume-manager + open device-manager. Verified: volume-probe
asserts the parse (bare-FAT volume at lba 0), neutral 10/10 across storage,
restart, display, logging, confinement — the volume manager now runs in
every boot and disturbs nothing.

test/qemu_test.py

@@ -755,6 +755,19 @@ CASES = [
      "timeout": 150,
      "expect": r"fat: mounted /volumes/usb[\s\S]*fat-test: ok",
      "fail":   r"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
+    # device manager, reads block 0, and parses the first volume out of it — the
+    # partition-table walk that used to live in the FAT engine, now above the
+    # driver where it belongs. Before V3a the service did not exist, so this line
+    # is absent.
+    {"name": "volume-probe",
+     "build_case": "fat-mount",
+     "smp":    4,
+     "timeout": 150,
+     "expect": r"volume-manager: volume 0x[0-9a-f]+ at lba \d+, \d+ blocks",
+     "fail":   r"DANOS-TEST-RESULT: FAIL"},
     # Phase 2b: mkdir/unlink through the mount. Reuses the fat-mount build — the
     # fat-test client, after listing, makes a directory, writes+reads a file inside
     # it, then removes the file, exercising the whole VFS -> fat mutation path.