fix(lanes): render undeclared lanes at their detected stage, quiet tmux polling noise

A lane whose sessions never call `ccam stage` had no declaration at all, so
its pipeline map stayed 100% pending even though detection was tracking real
progress. inferredDisplayStage renders such a lane AT its last detection
instead (display only — the DB's declared `stage` and detection-as-evidence
rules are untouched). Also stops tmux's own "no session" stderr noise from
spamming the journal on every poll.

server/lib/tmux.js

@@ -18,13 +18,18 @@ const { execFileSync } = require("node:child_process");
 
 // Test seam: swap the exec implementation so unit tests never invoke a real
 // tmux binary. Mirrors run-spawner.js's __injectChildForTest/__reset style.
-let execImpl = (args) => execFileSync("tmux", args, { encoding: "utf8" });
+// stderr is ignored, not inherited: every caller already wraps these in
+// try/catch and treats failure as "no session" — piping tmux's own
+// "error connecting to ..." noise to the process's stderr just spams the
+// journal on every poll while no tmux server is running yet.
+const EXEC_OPTS = { encoding: "utf8", stdio: ["pipe", "pipe", "ignore"] };
+let execImpl = (args) => execFileSync("tmux", args, EXEC_OPTS);
 
 function __setExecImpl(fn) {
   execImpl = fn;
 }
 function __reset() {
-  execImpl = (args) => execFileSync("tmux", args, { encoding: "utf8" });
+  execImpl = (args) => execFileSync("tmux", args, EXEC_OPTS);
 }
 
 function hasSession(name) {