block: medium presence — the medium_changed event and usb-storage as publisher (V2b)

The block protocol gains a pushed medium_changed event (present + a monotonic
change counter; presence only, never content). usb-storage becomes a
Subscribers provider and runs a slow TEST UNIT READY poll (1 s): success is
present, failure absent, and a transition bumps the counter and publishes.
This is the second removal trigger — the DEVICE stays while the MEDIUM leaves
(card readers, ATAPI trays) — which channel death cannot see
(storage-architecture.md, two triggers one lifecycle).

The subscriber is the volume manager (V3); until it exists the publish is a
no-op fan-out, so this commit is behaviour-neutral, and its end-to-end test
(eject -> medium_changed -> unmount/remount) lands in V4 with the real
consumer rather than a throwaway subscriber fixture (recorded sequencing).
Sense-key inspection to tell medium-absent from other transport errors is a
noted refinement; a clean eject reads correctly as not-ready.

Neutral: 12/12 across the block-serving surface, restart, confinement,
conformance, and logging.

library/protocol/block/block-protocol.zig

@@ -57,9 +57,27 @@ pub const DefineRange = extern struct {
     block_count: u64,
 };
 
+/// The `medium_changed` event payload: whether a medium is now present, and a
+/// monotonic counter so a subscriber that missed an edge still sees that
+/// SOMETHING changed. Pushed by a driver whose transport can tell medium from
+/// device (a card reader, an ATAPI tray): the device stays, the medium comes and
+/// goes. The volume manager consumes it into the same unmount/remount path it
+/// runs on device death — one lifecycle, two triggers
+/// (docs/file-system-development/storage-architecture.md). Presence only, never
+/// content: the driver reports that the medium changed, not what is on it.
+pub const MediumChanged = extern struct {
+    present: u8, // 1 present, 0 absent
+    _padding: u8 = 0,
+    _padding2: u16 = 0,
+    change_count: u32,
+};
+
 pub const Protocol = envelope.Define(.{
     .name = "block",
     .version = 1,
+    .events = &.{
+        .{ .name = "medium_changed", .payload = MediumChanged },
+    },
     .operations = &.{
         .{ .name = "geometry", .reply = Geometry },
         .{ .name = "read", .request = Transfer, .reply = Transferred },
@@ -88,4 +106,5 @@ pub const Protocol = envelope.Define(.{
 });
 
 pub const Operation = Protocol.Operation;
+pub const Event = Protocol.Event;
 pub const message_maximum: usize = Protocol.message_maximum;