A pure step remembers the governor's last change, so a reader can say "I lowered builders to 3 at 20:41" #6520

Part of #5930, section 3: the page says what the governor already did.

Done:

• [ ] New src/crew/governor_log.ts exports governorStep(prev, governed, now) → { last, changed }, where last is { from, to, why, at }; a reading with the same slot count changes nothing.
• [ ] governorSays(last) words it: "Builders 7 → 3 at 20:41: <why>", or "Builders back to 7 at 21:30" when it rises to the ceiling.
• [ ] Tests: lower, hold, raise, and the words for each. The governor module itself is not edited (the picker work in #6375 is in flight there).

Files: src/crew/governor_log.ts, test/governor-log.test.ts.

Before pushing: bun x tsc --noEmit and the new test.