volume-manager: partition parser takes a SectorReader; identity is a tagged Identity (S1)

The identity ladder's flag-day — no behavior change. partition.firstVolume stops
taking one preloaded block-0 slice and takes a SectorReader (a read-one-sector
fn), so it can reach GPT metadata at LBA 1 and each partition's VBR on demand
(the next commits). The u64 identity becomes Identity{rung,key,label}: key is the
id (the mount path derives from it), label is display metadata (empty at rung 4).
Identity equality is id-only (rung+key) — the label never enters it. Only rung-4
(MBR sig+index / bare-FAT index 0) is produced, byte-identical to before; the
four host tests port to a RAM-disk reader, and fat-mount/volume-probe/
volume-removal stay green.

system/services/volume-manager/volume-manager.zig

@@ -40,7 +40,7 @@ const Volume = struct {
     storage_device_id: u64, // the device-manager id this volume's provider serves
     base_lba: u64,
     block_count: u64,
-    identity: u64,
+    identity: partition.Identity,
     id: u64,
     filesystem_pid: u32 = 0,
 };
@@ -169,7 +169,7 @@ fn spawnFilesystem(v: *Volume) void {
     }
     v.filesystem_pid = pid;
     fs_spawn_ns = time.clock();
-    std.log.info("volume 0x{x} -> {s} (pid {d}), lba {d}, {d} blocks", .{ v.identity, filesystem_binary, pid, v.base_lba, v.block_count });
+    std.log.info("volume 0x{x} -> {s} (pid {d}), lba {d}, {d} blocks", .{ v.identity.key, filesystem_binary, pid, v.base_lba, v.block_count });
 }
 
 /// Schedule a fat restart after backoff; the poll loop performs it once due.
@@ -204,12 +204,19 @@ fn bringUpVolume() void {
         _ = ipc.close(device.endpoint);
         return;
     };
-    if (!device.read(0, 1, bounce.physical)) {
-        _ = ipc.close(device.endpoint);
-        return;
-    }
-    const sector: [*]const u8 = @ptrFromInt(bounce.virtual);
-    const found = partition.firstVolume(sector[0..512], geometry.block_count) orelse {
+    const ProbeReader = struct {
+        device: block.Device,
+        fn readSector(context: *anyopaque, lba: u64, buffer: *[partition.sector_bytes]u8) bool {
+            const self: *@This() = @ptrCast(@alignCast(context));
+            if (!self.device.read(lba, 1, bounce.physical)) return false;
+            const src: [*]const u8 = @ptrFromInt(bounce.virtual);
+            @memcpy(buffer, src[0..partition.sector_bytes]);
+            return true;
+        }
+    };
+    var probe = ProbeReader{ .device = device };
+    const reader = partition.SectorReader{ .context = &probe, .readFn = ProbeReader.readSector };
+    const found = partition.firstVolume(reader, geometry.block_count) orelse {
         if (!logged_no_volume) {
             _ = logging.write("volume-manager: storage present but no recognizable volume\n");
             logged_no_volume = true;