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/httpapi/server.go

@@ -12,11 +12,12 @@ import (
 
 type Server struct {
 	Agg *state.Aggregator
+	Hub *Hub
 	Log *slog.Logger
 }
 
 func New(agg *state.Aggregator, log *slog.Logger) *Server {
-	return &Server{Agg: agg, Log: log}
+	return &Server{Agg: agg, Hub: NewHub(), Log: log}
 }
 
 // Handler returns an http.Handler with all kamado routes mounted under /api.
@@ -27,7 +28,9 @@ func (s *Server) Handler() http.Handler {
 	mux.HandleFunc("GET /api/users", s.users)
 	mux.HandleFunc("GET /api/workers", s.workers)
 	mux.HandleFunc("GET /api/clients", s.clients)
+	mux.HandleFunc("GET /api/blocks", s.blocks)
 	mux.HandleFunc("GET /api/snapshot", s.snapshot)
+	mux.HandleFunc("GET /api/ws", s.handleWS)
 	return mux
 }
 
@@ -83,3 +86,7 @@ func (s *Server) workers(w http.ResponseWriter, r *http.Request) {
 func (s *Server) clients(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, s.Agg.Snapshot().Clients)
 }
+
+func (s *Server) blocks(w http.ResponseWriter, r *http.Request) {
+	writeJSON(w, http.StatusOK, s.Agg.Blocks())
+}