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.

library/protocol/volume-manager/volume-manager-protocol.zig

@@ -0,0 +1,36 @@
+//! The volume-manager protocol (docs/file-system-development/storage-architecture.md):
+//! what a filesystem service says to the volume manager over
+//! `/protocol/volume-manager`. Defined through the envelope, so every packet
+//! begins with the folded `Header`.
+//!
+//! One verb. A filesystem the volume manager spawned announces itself with the
+//! volume id it was given as argv[1] (folded into `Header.target`); the reply
+//! carries that volume's block channel — already range-confined to the
+//! filesystem's badge — as the call's returned capability. The filesystem never
+//! finds its storage by name and never sees the whole device; establishment is
+//! by lineage, exactly as a driver reaches its controller (communication.md
+//! "Establishment: two planes"). No channel in the reply means the volume is not
+//! ready yet — retryable, never a verdict.
+
+const envelope = @import("envelope");
+
+pub const version: u16 = 1;
+
+/// The filesystem's handshake. Carries only its protocol version; the volume it
+/// serves is `Header.target`, and the block channel it needs comes back as the
+/// reply's capability.
+pub const Hello = extern struct {
+    version: u16 = version,
+    _padding: u16 = 0,
+};
+
+pub const Protocol = envelope.Define(.{
+    .name = "volume-manager",
+    .version = 1,
+    .operations = &.{
+        .{ .name = "hello", .request = Hello },
+    },
+});
+
+pub const Operation = Protocol.Operation;
+pub const message_maximum: usize = Protocol.message_maximum;