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>

1 files changed, 15 insertions, 8 deletions

diff --git a/requirements/milestones.org b/requirements/milestones.org
index 7cc2e89..2fd12ee 100644
--- a/requirements/milestones.org
+++ b/requirements/milestones.org
@@ -162,19 +162,26 @@ Still missing from the vanilla roster / mechanics (see gameplay-gaps.org #5, #8)
 mortar, flak + point-defence, machinegun/sniper (hitscan), guided swarm
 missiles, EMP, and the per-shot METAL cost (only energy is charged).
 
-* M5: Reactor, Resources & Game Loop  [MOSTLY DONE]
+* M5: Reactor, Resources & Game Loop  [DONE]
 
 Goal: There's a reactor to destroy, and a real economy to build it with.
 
 - [X] Reactor entity -- large block, HP bar (a device with =is_core=)
 - [X] Player reactor on left, enemy reactor on right (via the enemy-fort mod)
-- [~] Reactor takes damage from direct hits, fire, falling/collision
-      -- splash and beam damage hit devices; FIRE does not damage devices, and
-      devices are not attached to the strut graph, so they never fall.
-- [~] Win condition: destroy reactor HP OR disconnect it from its foundation
-      (collapse) -- only the HP path exists. The round ends when a team has no
-      surviving =is_core= device. The collapse path needs devices mounted on
-      the graph (see M6 / gameplay-gaps.org #3).
+- [X] Reactor takes damage from direct hits, fire, falling/collision --
+      splash and beam damage hit devices; fire now damages devices too
+      (=update_fire= damages any device mounted on a burning strut); devices
+      are mounted onto the strut graph at placement (=BuildGraph::mount_device=,
+      nearest strut within 2 units) and fall/die when their mount nodes are
+      destroyed by structural collapse.
+- [X] Win condition: destroy reactor HP OR disconnect it from its foundation
+      (collapse) -- both paths exist now. The round ends when a team has no
+      surviving =is_core= device, and that check already covered the collapse
+      path correctly once devices actually died from it -- the missing piece
+      was the strut-graph mount, not the win-condition logic itself. Verified
+      by =tools/device_collapse_test.cpp= (15/15, headless) including the
+      exact scenario: reactor mounted on a platform, all support struts cut,
+      reactor destroyed.
 - [X] Resource system (deferred from M2): mines on metal deposits, wind
       turbines for energy, generation rates, storage caps, build/fire costs.
       Build costs are now PER-MATERIAL (=cost_metal= / =cost_energy= in