control board: stale registrations and a launch-test leak (#1504)

Two defects in 69f99323, reported by the professor session and verified.

The darkwing launch test's flock-contention spawn ran without the fixture
config, so launch.sh re-entered scripts/mosaic against the real data root
and wrote fixture records for darkwing, dewey and filbert there. That
spawn now names the fixture config, and both launch test files set
MOSAIC_CONFIG to a nonexistent path and clear MOSAIC_LAUNCH_REGISTERED
process-wide, so a spawn that forgets fails instead of polluting.

A registration is written before the launch script's own checks, so a
refused launch left a record with a dead pid that the board honoured. The
scanner now probes the recorded pid (pidAlive, signal 0); a gone pid makes
the record stale: still on the Registered line with alive false, derived
task, project and workspace win, index gains registrationStale, CLI
summary gains a stale count.

Fleet launchers marked not planned per Jason. Board 90/90, seat 15/15,
launch scripts 5/5. Sonnet review APPROVED.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

packages/seat/src/cli.mjs

@@ -4,7 +4,7 @@
 //                 [--harness NAME] [--repo PATH] [--config PATH] [-- args...]
 //   mosaic seat task <seat> <text> [--layout repo|fleet|unknown] [--config PATH]
 //
-// `launch` writes <dataRoot>/seats/<seat>/registration.json, then replaces
+// `launch` writes <dataRoot>/seats/<layout>/<seat>/registration.json, then replaces
 // itself with the seat's launch.sh, unchanged, with everything after `--` as
 // its arguments and MOSAIC_LAUNCH_REGISTERED set to the record path. A launch
 // script that sees that variable is already registered and must not call