ops: Signal disk-tier attach pending vs ready

Large disk caches can look memory-only/broken during async DiskFS attach.
Expose disk_tier_ready in /metrics, pending/ready logs for disk-only and
mixed modes, and X-SteamCache-Disk-Tier on /lancache-heartbeat. GetMetrics
skips blocking disk.Size() while attach is pending so /metrics stays usable.

Closes #33

README.md

@@ -83,6 +83,7 @@ curl -s -i http://localhost/lancache-heartbeat
 | `cache_hits` / `cache_misses` / `hit_rate` | Whether later requests were served from cache |
 | `memory_cache_hits` / `disk_cache_hits` | Which tier served the hits |
 | `total_requests` / `errors` | Volume and failures |
+| `disk_tier_ready` | `0` while disk slow-tier attach pending; `1` when attached, or when no disk configured (N/A — not waiting) |
 
 A first pass through new content is mostly misses (`hit_rate` near 0). Repeat the same content and `cache_hits` / `hit_rate` should rise.
 
@@ -94,6 +95,8 @@ curl -s -i http://localhost/lancache-heartbeat
 
 Use GET (`curl -i`), not HEAD (`curl -I`): the server only accepts GET.
 
+Heartbeat also returns `X-SteamCache-Disk-Tier: pending|ready|disabled` (`disabled` = memory-only / no disk; `pending`/`ready` = disk configured attach state).
+
 These are the cache process's own `/metrics` and `/lancache-heartbeat` endpoints. There is no separate metrics daemon.
 
 If you changed `listen_address`, point curl at that host:port instead. For a full SteamPrefill validation workflow (small caches, coalescing, GC), see [Validating Full Functionality](#validating-full-functionality-with-external-tools).
@@ -287,6 +290,9 @@ See `config.Validate()` and `steamcache.New` error paths. This ensures the LAN a
 - The explicit startup guard (reduce size if pre-existing on-disk > cap) runs as the literal last step of bg init, before the barrier opens.
 - Add a note for operators: very large disk caches (tens/hundreds GB with millions files) may show extended "memory-only or no-cache" behavior at startup (seconds to minutes depending on storage speed); this is by design for responsiveness.
 - Godoc on `disk.New` and `DiskFS.Size` expanded with the barrier/attach behavior.
+- Startup logs: Info "Disk slow tier attach pending..." then later "Disk slow tier attached (...)" for disk-only and mixed modes.
+- `/metrics` exposes `disk_tier_ready` 0/1 and stays responsive during attach (GetMetrics does not block on Size while pending).
+- `/lancache-heartbeat` header `X-SteamCache-Disk-Tier` mirrors that state.
 
 #### Garbage Collection Algorithms