feat(art): integrate Synty vehicles and props

[ralyodio]

Ship the static-mesh half of the Synty pack (handoff step 4), which needs none of the character retarget: two vehicles (armoured car, technical) and five props (barrel, barrel stack, ammo box, T-wall barrier, water tank).

New converter tools/art/blender/import_synty_prop.py: applies the importer's metre-scale transform (the static FBX carries unit metadata; only the skeletal FBX is centimetres), recentres to ground, names material slots by mesh, welds, and decimates the vehicles.

Textures stay cheap. The Land_Vehicles atlas ships as one desert variant (synty_vehicles.webp, 76 KB) bound to every vehicle, since the pack's ten variants share a UV layout. Props reuse the character atlas and add no texture at all. Vehicles decimate ~1.5 MB -> ~0.5 MB with no visible loss, verified by preview render. Shell budget 5.38 -> 6.33 MB, under the 9 MB guard.

Placed as cosmetic set-dressing in the spawn zones and back corners, non- colliding and out of the three combat lanes, so world.ts rule 1 still holds for the play space. Promoting any to real cover is a separate map.ts change.

[ralyodio]

Superseded and stale. This branch was cut from 9885785, before #27 (vehicles + props) and #28 (weapons) landed on main. main already carries byte-identical vehicle/prop GLBs, the synty_vehicles atlas, and the import_synty_prop.py converter this branch adds — and additionally has the weapons work. Merging #29 would regress main (remove wep_*/synty_weapons and revert assets.ts/opponents.ts/viewmodel.ts). Nothing here is missing from main, so closing.