chore(controller): clusterconfig-schema-split task 5.3 — single-node/peers-override/EINVAL live-verified

Live-verified all three cluster_open_listener code paths not exercised by
the 3-node soak: no --peers binds hardcoded default 9990; a single
self-referencing --peers entry binds its custom port instead; --peers
given but local name absent still refuses to start with -EINVAL, unchanged.
Independently reproduced by the reviewer. This closes the actual
regression risk this spec's design was built around.

Review also found (bug-1457, logged, not fixed here) that pre-spec
single-node startup was already broken with -EINVAL before this spec
began, due to an unrelated myapp_run listener-open/config-load ordering
issue -- task 2.1's rewrite incidentally repaired it. research.md updated
to record this against the spec's original preservation premise.

.kiro/specs/clusterconfig-schema-split/tasks.md

@@ -84,7 +84,7 @@
   - _Requirements: 3.3, 5.4_
   - _Depends: 4, 5.1_
 
-- [ ] 5.3 Single-node startup, self-referencing-peers port override, and preserved error-path verification
+- [x] 5.3 Single-node startup, self-referencing-peers port override, and preserved error-path verification
   - Start a controller with no `--peers` flag and confirm it binds its listener to the hardcoded default port (9990) from task 2.1
   - Start a controller with a single self-referencing `--peers <name>=<host>:<port>` entry (own `-n`/`--myname` matching, custom port) and confirm it binds to that port instead of the default — this is the sole replacement mechanism for a non-default single-node port under this spec
   - Start a controller with a `--peers` list that does not contain the controller's own name and confirm it still refuses to start with the existing `-EINVAL` error, unchanged from pre-spec behavior