Phase 1a: cluster module + LAN-first peer probe

First cut of the v2 distributed FS. Captures the architecture design
in ARCHITECTURE-v2.md and lands the smallest useful new capability:
probing cluster peers with a LAN-first policy so subsequent transport
+ gossip layers (Phase 1b, 1c) can build on a real routing decision.

New:
- ARCHITECTURE-v2.md: zones (fabric-10g/lan-1g/roaming), tier
  lifecycle (hot/warm/cold), fingerprint-keyed build cache design,
  smart-clean policy, phase plan, explicit non-goals.
- claw-store/src/cluster.rs: RouteKind, RouteWinner, LanFirstProbe.
  LAN 200ms timeout, Tailscale 500ms fallback. 8 tests use real
  TCP listeners on 127.0.0.1 (no mocks); cover happy path,
  fall-through, both-fail, single-address, and elapsed reporting.
- claw-store/src/config.rs: ClusterConfig + PeerEntry with
  validation (bind-address presence, no duplicate peer names,
  per-peer reachable address required). Optional at top level so
  pre-v2 configs still load unchanged. 6 new tests.
- claw-store/src/main.rs: `claw-store cluster-probe <peer>` CLI
  subcommand that reads config, resolves the peer, probes, prints
  the winning route + elapsed time.

All 16 new tests pass. Existing 45 pass. Sole failure
(hot::tests::test_project_target_size_bytes) is a pre-existing
macOS-only issue with `du -sb`; Linux CI unaffected.

Follow-on Phase 1 cuts (subsequent sessions):
- 1b: chitchat SWIM gossip for live membership state
- 1c: quinn QUIC transport with fleet-CA mTLS
- 1d: `claw-store cluster status` — live membership view

Every file well under the 1300-line ceiling
(cluster.rs 268, config.rs 428, main.rs 450).

claw-store/src/actions.rs

@@ -126,6 +126,7 @@ mod tests {
             cold: None,
             replication: None,
             peer: None,
+            cluster: None,
             api_token: None,
         }
     }