feat(mpc): identify accepted plan decisions

[frahlg]

Summary

• Assign a Core-owned UUID only after the latest replan passes the publish gate.
• Use (decision_id, slot_start_ms) as the stable planned-action key.
• Carry the ID through the live plan, diagnostic JSON/SQLite, restart restore, and SlotDirective.
• Reject overlapping input slots before any solver runs.
• Reject solver output unless its action count, start, and length match every input slot.
• Keep legacy diagnostics valid with an empty ID.

Scope

This changes only Core MPC files. It does not change the external optimizer wire format, database schema, control dispatch, driver commands, or measured-outcome tracking. It adds the join key needed for that later work.

Verification

• make verify-all
• Focused decision, chronology, restore, persistence, and latest-wins tests: 200 runs
• The same focused concurrency paths with the race detector: 25 runs
• Three final read-only reviews: no P0-P3 findings

Part of #885.