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/bitcoind/rpc.go

@@ -113,6 +113,15 @@ func (c *RPC) GetBlockchainInfo(ctx context.Context) (*BlockchainInfo, error) {
 	return &out, nil
 }
 
+// GetBlockHash returns the block hash at the given height.
+func (c *RPC) GetBlockHash(ctx context.Context, height int64) (string, error) {
+	var out string
+	if err := c.Call(ctx, "getblockhash", []any{height}, &out); err != nil {
+		return "", err
+	}
+	return out, nil
+}
+
 // NetworkHashPS returns the network hashrate at the given block height.
 // `blocks` is a window (default 120). Pass -1 to use the default.
 func (c *RPC) GetNetworkHashPS(ctx context.Context, blocks, height int) (float64, error) {