gui/notify: add mandates_expiring_early notification

[Alistair-Afton]

Adds an opt-in mandates_expiring_early notification that warns when a production mandate has less than 3 months remaining — roughly the midpoint of the typical ~6 month mandate duration. The existing mandates_expiring notification still fires within ~1 month of the deadline, so enabling both gives escalating warning tiers, as suggested in the issue.

The early notification counts all Make mandates within its window (including imminent ones), so enabling it alone never hides a near-deadline mandate. It is disabled by default to avoid extra overlay noise for players who don't want it.

Implementation notes:

• mandate.timeout_counter increments once per 10 frames, so 3 months = 10080 counter units and the existing "1 month" threshold of 2500 is ~3 weeks.
• The shared mandate-counting loop is factored into a helper used by both notifications.
• New in-engine regression test (test/notify.lua) fabricates mandates at all three threshold windows and asserts both notifications report the right counts. 1/1 tests, 8/8 checks pass in-game on 53.16.

Closes DFHack/dfhack#5767