feat(controller-status): add election-time schema fields + ha_state_t (task 7.1)

Adds CtrlStatus.electedAtMs@14 and HeartbeatMsg.electedAtMs@4 to the
capnp schema, their hand-authored typemap.h counterparts, and a new
ha_state_t.last_known_elected_at_ms field alongside last_known_committed_lsn.
Schema/state plumbing only -- nothing stamps or consumes these fields yet
(tasks 7.2-7.5). Codec round-trip tests confirm both new fields survive
encode/decode unchanged.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RLSNJhNoULvVcUhXEFafQC

.kiro/specs/controller-status/tasks.md

@@ -92,7 +92,7 @@
 ## 7. Delta: primary election term + time (Req 12)
 > Sequential — the election-time stamp touches the same HA promotion/heartbeat control flow as tasks 2.2/6.2 (ha.c); no `(P)`. Read-only reporting only: this task must not change how elections are conducted, how term advances, or how/when role transitions to primary — every write here is an additional observational stamp at an already-decided instant. `electedTerm` needs no new field (design.md Locked Decision 1); it is a CLI-side derivation added in 7.5, not new state.
 
-- [ ] 7.1 Add the election-time schema fields and controller state
+- [x] 7.1 Add the election-time schema fields and controller state
   - Add a wall-clock election-time field to the status reply payload (next free ordinal, append-only) and to the heartbeat message the primary already sends to standbys (next free ordinal).
   - Add the corresponding piece of HA state that holds the currently-known primary's election time, alongside the existing learned-committed-position state.
   - Add the matching fields to the hand-authored C structs backing these two payloads (this schema's `$C.mapname` types are NOT auto-generated — the generated codec references hand-written typedefs in `typemap.h`, per tasks 1.1/2.1/6.1's established convention) and verify each new field field-for-field against the regenerated codec.