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/logmon/inode_unix.go

@@ -0,0 +1,17 @@
+//go:build unix
+
+package logmon
+
+import (
+	"os"
+	"syscall"
+)
+
+// inodeOf returns the inode number for a FileInfo on unix systems, or 0
+// if the underlying stat is unavailable. Used to detect log rotation.
+func inodeOf(fi os.FileInfo) uint64 {
+	if st, ok := fi.Sys().(*syscall.Stat_t); ok {
+		return st.Ino
+	}
+	return 0
+}