volume-manager: the flip — fat is spawned, confined, and handed its channel (V3b)

The load-bearing step. The FAT service stops acquiring its own volume: the
volume manager spawns it (per volume), defines its partition range on the
storage driver BEFORE it runs, and answers its startup hello with the
range-confined block channel over a new volume-manager protocol. fat never
finds its storage by name and never sees the whole device — establishment
by lineage, one layer up from the driver tree.

- New library/protocol/volume-manager: one verb, hello(volume-id) -> the
  block channel as the reply capability (the P0 reply-cap path).
- The volume manager becomes the confinement CONTROLLER: it defines the first
  range on usb-storage, so no other party can confine a filesystem. It
  supervises the filesystems it spawns and respawns one on death (the reap-
  and-rebuild the device manager proved, one layer up).
- fat: drops acquireVolume(device-manager); hellos the volume manager for its
  channel; reads its volume id from argv[1]. main takes process.Init now.
- init.csv no longer spawns fat (the volume manager does); protocol.csv
  rewires fat to be supervised by the volume manager (bind vfs, open
  volume-manager) and drops fat open device-manager.
- The block-range fixture boots registry + device-manager only (not the full
  tree), so the volume manager is absent and the fixture stays the sole
  confinement definer — otherwise the volume manager would take the
  controller first and refuse it.

Verified end to end (VM probes -> spawns fat -> confines it -> hands over the
channel -> fat mounts) and neutral: 18/18 across the fat family, logging,
shutdown, both IOMMU variants, usb restart, vfs, conformance, confinement.

system/services/fat/build.zig

@@ -10,9 +10,9 @@ pub fn build(b: *std.Build) void {
         .name = "fat",
         .root_source_file = b.path("fat.zig"),
         .imports = &.{
-            "block",     "channel",             "device-manager-protocol",
-            "driver",    "envelope",            "file-system-harness",
-            "ipc",       "logging",             "memory",
+            "block",     "channel",  "envelope", "file-system-harness",
+            "ipc",       "logging",  "memory",   "process",
+            "time",      "volume-manager-protocol",
         },
     });
     b.installArtifact(exe);