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/configuration/protocol.csv

@@ -56,7 +56,9 @@
 /system/services/input,                      /system/services/init,           bind,       input
 /system/services/device-manager,             /system/services/init,           bind,       device-manager
 /system/services/volume-manager,             /system/services/init,           bind,       volume-manager
-/system/services/fat,                        /system/services/init,           bind,       vfs
+# fat is spawned and supervised by the volume manager now, not init — the volume
+# manager confines it to its partition and hands it the block channel.
+/system/services/fat,                        /system/services/volume-manager, bind,       vfs
 /system/services/display,                    /system/services/init,           bind,       display
 
 # The discovery service ships under one neutral name per firmware (docs/discovery.md);
@@ -95,14 +97,13 @@
 # ============================================================================
 
 # --- init's own services ----------------------------------------------------
-# fat reaches the device manager to be routed to its volume's block provider
-# (block is not a name — see the bind section); the compositor reaches the
-# scanout its driver announced, its own endpoint (the mouse-listener thread
-# opens /protocol/display like any other client — threads share no handles),
-# and the input stream that moves the cursor.
-/system/services/fat,                        /system/services/init,           open,       device-manager
+# fat reaches the volume manager to be handed its volume's block channel
+# (range-confined); the compositor reaches the scanout its driver announced, its
+# own endpoint (the mouse-listener thread opens /protocol/display like any other
+# client — threads share no handles), and the input stream that moves the cursor.
+/system/services/fat,                        /system/services/volume-manager, open,       volume-manager
 # The volume manager reaches the device manager to be routed to each storage
-# provider's block channel, the same lineage acquisition fat makes today.
+# provider's block channel, then confines a filesystem to each volume.
 /system/services/volume-manager,             /system/services/init,           open,       device-manager
 /system/services/display,                    /system/services/init,           open,       scanout
 /system/services/display,                    /system/services/init,           open,       display