Keep coastal planning alive and prefer 400 m offshore

[woensug-choi]

A 120-second controller watchdog repeatedly cancelled terrain preparation whose provider deadline is 620 seconds. Preserve pending work within one 660-second preparation budget and consume completed results before restarting. Start with a 6 km fine grid within 1.5 km of shore, expand once when no checked connection exists, and retain compatible checked runway during the 400 m preference migration.

Validation: all 89 routing tests passed, including pending-work survival, expired-work cancellation, fine-grid expansion, and migration continuity. A replay on the actual Quy Nhon terrain found a checked coastal route at 387 m endpoint standoff, with 3.29 km progress toward Singapore and unchanged land/depth checks.