feat(controller-status): source election time in ctrlstatus_fill (task 7.4)

ctrlstatus_fill now unconditionally reads ha->last_known_elected_at_ms
into the reply's elected_at_ms field -- one atomic-acquire load, no
role-branch, mirroring how the handler already reports raw term
unconditionally. Serves both a self-stamped primary and a
heartbeat-learned standby from the same field. "Unknown" display
gating is deliberately left to the client command (task 7.5), not
this handler.

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

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

@@ -116,7 +116,7 @@
   - _Boundary: heartbeat (send + receive)_
   - _Depends: 7.1_
 
-- [ ] 7.4 Source the election time in the status handler
+- [x] 7.4 Source the election time in the status handler
   - TDD (red first): the status handler reports a non-zero election-time equal to the responder's own learned value (self-stamped on a primary, heartbeat-learned on a standby); a standby seeded with a learned election-time (simulating an already-processed heartbeat) reports that learned value, not zero; the handler still mutates no state.
   - Read the learned election-time into the status reply unconditionally (no role-based branching, no "unknown" gating in the handler itself — that display decision belongs to the client command in 7.5, mirroring how the handler already reports raw term unconditionally regardless of whether a primary is currently known).
   - Observable: the new unit tests go red → green; a standby seeded with a learned election-time reports that value instead of zero; the no-mutation assertion holds.