multithreaded collision fixes

[Goober5000]

Three commits, three four fixes to the multithreaded collision code. These correspond to items 1, 2, 3, and 5 from Claude's audit report.

1. rename misleading variable check_again to never_check_again

The first element of collision_result means "never check again" per the typedef comment, but was destructured as check_again (opposite meaning). Rename to never_check_again to match the actual semantics and prevent future confusion.

2. fix data races in ship-weapon collision threading

Two thread-safety bugs in ship_weapon_check_collision, which runs on
worker threads:

1. update_danger_weapon() wrote to Ai_info[] (danger_weapon_objnum,
   danger_weapon_signature) without synchronization. Multiple weapons
   targeting the same ship would race on the same Ai_info entry.

2. Homing missile detonation logic wrote wp->lifeleft and
   wp->weapon_flags directly on the Weapons[] global array.

Fix: defer both writes to the main-thread post-processing phase via
new flags in ship_weapon_collision_data. Also convert the collision
data from a 6-element tuple to a named struct for readability.

3. fix data race in ship-ship collision submodel checking

ship_ship_check_collision() toggled pmi->submodel[].collision_checked
flags on the heavy ship's polymodel_instance to selectively enable/
disable submodel collision checks. When two collision pairs shared the
same heavy ship, worker threads would race on the same pmi entries.

Fix: add a collision_checked_override pointer to mc_info that lets
callers provide a thread-local array of collision_checked values.
model_collide() uses the override when set, falling back to the
pmi->submodel[] field otherwise. ship_ship_check_collision() now
builds a local SCP_vector and passes it via the override,
completely avoiding writes to shared polymodel_instance state.

Also added @BMagnu 's fix for bug 4:

4. Ensure threads have spun down before continuing non-threaded execution