Phase 2b: log tailer, block history, stdlib WebSocket push

Adds real-time block detection via ckpool log tailing and a push
channel for the upcoming Svelte UI, all stdlib-only:

- logmon.Tailer follows ckpool.log with rotation/truncation survival
  (inode + size tracking) and parses "Solved and confirmed block N"
  into BlockEvent values.
- state.Aggregator grows a 256-entry block ring, an OnRefresh hook,
  and IngestBlockEvents which best-effort enriches events with the
  block hash via bitcoind getblockhash.
- httpapi.Hub implements RFC 6455 from scratch (SHA1 handshake,
  unmasked text frames out, masked frames in, ping keepalive,
  per-client write mutex, slow-client drop) so we don't pull in a
  ws dependency before we can go mod tidy.
- New routes: GET /api/blocks and GET /api/ws. Snapshot pushes fire
  on every poll tick and immediately on block-solve.

ZMQ hashblock subscription and SQLite persistence are deferred to
Phase 2b.5 once the s9pk packaging repo exists and we have a real
build environment for adding Go deps.

api/internal/state/aggregator.go

@@ -36,6 +36,9 @@ type Snapshot struct {
 	Chain                *bitcoind.BlockchainInfo `json:"chain"`
 	NetworkHashrateHs    float64                  `json:"network_hashrate_hs"`
 
+	// Recent found blocks (in-memory history; persisted in Phase 2b.5).
+	RecentBlocks []BlockRecord `json:"recent_blocks,omitempty"`
+
 	// Health
 	CKPoolOK  bool   `json:"ckpool_ok"`
 	BitcoinOK bool   `json:"bitcoin_ok"`
@@ -49,8 +52,13 @@ type Aggregator struct {
 	Interval time.Duration
 	Log      *slog.Logger
 
-	mu   sync.RWMutex
-	snap Snapshot
+	// OnRefresh, if set, is called (non-blocking) after each snapshot
+	// refresh. Used by the WebSocket hub to push updates to clients.
+	OnRefresh func(Snapshot)
+
+	mu     sync.RWMutex
+	snap   Snapshot
+	blocks []BlockRecord
 }
 
 func New(ck *ckpool.Client, rpc *bitcoind.RPC, interval time.Duration, log *slog.Logger) *Aggregator {
@@ -132,6 +140,18 @@ func (a *Aggregator) refresh(ctx context.Context) {
 	}
 
 	a.mu.Lock()
+	// Attach current block history so /api/snapshot and WebSocket
+	// pushes carry the same view.
+	if len(a.blocks) > 0 {
+		next.RecentBlocks = make([]BlockRecord, len(a.blocks))
+		copy(next.RecentBlocks, a.blocks)
+	}
 	a.snap = next
+	cb := a.OnRefresh
+	pushed := next
 	a.mu.Unlock()
+
+	if cb != nil {
+		cb(pushed)
+	}
 }