Turn the client into a background daemon; add per-chunk hash verification

Daemon (src/daemon): a Unix-socket control server holds every room this
machine is participating in. "share send"/"share <code>" resolve paths to
absolute, ask the daemon to join (auto-starting it if needed), and exit
immediately instead of blocking. "share stop" leaves every room and shuts
the daemon down; a stale socket from a crashed daemon self-heals on next
start.

swarm.Client is now a Start/Wait/Stop handle instead of a single blocking
Run, so the daemon can hold many rooms concurrently.

Also added per-chunk sha256 verification: the original seeder publishes a
hash for every chunk alongside the whole-file hash, and each chunk is
checked against it the moment it arrives — before it's written to disk or
reshared to other peers — rather than only catching corruption after the
whole file is assembled.

src/daemon/protocol.go

@@ -0,0 +1,25 @@
+package daemon
+
+// request/response are the only vocabulary spoken over the control
+// socket between a CLI invocation and the background daemon.
+type request struct {
+	Action string       `json:"action"` // "join" or "stop"
+	Join   *joinRequest `json:"join,omitempty"`
+}
+
+// joinRequest asks the daemon to start participating in a room. Paths
+// must already be absolute — the daemon may have been started long ago
+// from a different working directory than whichever CLI invocation is
+// submitting this request.
+type joinRequest struct {
+	RelayAddress string   `json:"relay"`
+	Code         string   `json:"code"`
+	Files        []string `json:"files,omitempty"`
+	OutDir       string   `json:"out,omitempty"`
+	OneTime      bool     `json:"oneTime,omitempty"`
+}
+
+type response struct {
+	OK    bool   `json:"ok"`
+	Error string `json:"error,omitempty"`
+}