[microoptimize] Default to collidefirebase=true if no firebases

Weapon `collideFirebase` defaults to `false`, so marines can shoot out of a bunker while sitting inside its collision volume. This is a good default.

However:
* if all the various `collideXYZ` tags are set to `true` for a weapon, collision check takes a fast path that skips them all and just assumes a collision.
* all other `collideXYZ` already default to `true`, `collideFirebase` is the only one that defaults to `false`.
* most games don't have firebases, so the default is a micropessimisation.

The engine could detect on load if there aren't firebases and set `collideFirebase` to false on all weapons.

No file or test is named. Trace weapon collision handling and the engine's load-time detection of available firebases, then confirm how the collideXYZ fast path treats collideFirebase. Done means the no-firebase case takes the intended fast path while collision behavior remains correct when firebases exist.

Written by the indexing model from the issue text.