Mount devices on the strut graph so cutting supports destroys them

Implements win-condition path (c): destroy the enemy reactor by cutting
its supports so it falls. Previously devices sat at fixed map coordinates,
structurally immune to collapse.

- BuildGraph gains MountedDevice tracking: mount_device/unmount_device,
  is_device_alive/get_device_hp/damage_device, nearest_device, with a
  break_node index-shift fix so mount references stay correct as other
  geometry on the same fort is destroyed. update_fire now also damages
  any device mounted on a burning strut.
- app.cpp mounts devices onto the nearest strut (within 2 units) at both
  map-load and interactive placement, and after each tick's structural
  collapse pass, zeroes a device's HP if its mount didn't survive -- which
  flows through the existing win-condition/cleanup code unchanged.
- tools/device_collapse_test.cpp: 15 headless tests covering collapse,
  partial support loss, splash/beam/fire-through-mount damage, multiple
  devices, index stability, and clean unmount.

Verified: full test suite passes (15/15), sim_check shows zero regressions
across all 3 maps.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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