Bare JSON scalar on the MCPL wire kills the host (uncaught TypeError in the line dispatcher) #217

Severity: P1 — remote host-kill on the deploy runtime.

src/mcpl/server-connection.ts (setupMessageRouting, ~line 1117): the try/catch covers only JSON.parse; the first use of the parsed value is 'id' in msg. A wire line that parses to a non-object — null, 42, "hi", true — throws TypeError: Cannot use 'in' operator to search for 'id' in null out of the transport's 'line' listener.

Verified end-to-end with the real StdioTransport + McplServerConnection and a child printing null\n followed by a valid notification: uncaught exception, host exit 1, the second line never delivered — identical on node 20 and bun 1.3.14 (connectome-host's pinned runtime; it installs no uncaughtException handler). Any connected MCPL server can terminate the host and every resident with a single line.

Pre-existing at least as far back as 03c31d9 — not introduced by #208, but it is the same containment boundary; see the ask in my #208 review (two-line fix + revert-goes-red test: guard non-object messages, last-resort try/catch around the post-parse dispatch, test sending null then a valid notification).

Related, lower severity, found in the same read: mcpl/manifestChanged and tools/list_changed sent as Requests (with an id) are dispatched to listeners that take no responder, so the request is never answered and a conforming server hangs (§6.6 answer-or-error).

Found during review of #208; probes by both panel Wefts, reproduced independently.