Refactor selection screen data storage to remove fixed arrays

[Goober5000]

Convert the mission ship/weapon select screens' per-class UI state from dense MAX_SHIP_CLASSES / MAX_WEAPON_TYPES arrays to sparse containers keyed by loadout-pool membership, matching the Ss_pool / Wl_pool maps they shadow:

ss_icon_info Ss_icons_teams[][MAX_SHIP_CLASSES] -> SCP_map<int, ss_icon_info>[]
wl_icon_info Wl_icons_teams[][MAX_WEAPON_TYPES] -> SCP_map<int, wl_icon_info>[]
wl_ship_class_info Wl_ships[MAX_SHIP_CLASSES] -> SCP_map<int, wl_ship_class_info>
int Plist[] / Slist[] + manual size counters -> SCP_vector

The ship-select active list is replaced outright by a vector of ship classes rebuilt on each interface sync, plus the existing scroll offset. active_list_remove() was dead code and is deleted.

Behavior fixes required by the conversion:

• wl_set_disabled_weapons now computes usability flags for the pool
  weapons plus every weapon currently in a slot's banks, instead of all
  weapon classes. (Bank weapons are not guaranteed pool members.)

• The two mouse-region handlers that read Plist[Plist_start+index] for
  raw indices 0..3 now bounds-check against the list size (as does
  ss_get_ship_class_from_list).

• wl_unload_icons and the ss unloaders early-return on a null team
  pointer instead of asserting and dereferencing.

The ship side loads icons for every pool class including exhausted 0-count entries while the weapon side loads only positive counts, preserving the existing asymmetry.

Also:

Fix bitmap and model handle leakage in the ship/weapon select screens, and make release symmetric with load:

• Initialize each team only once in ship_select_common_init
• Unload every team's map in ss_unload_team_icons, not just the active one.
• Add a per-team unload helper and use it in the load-all functions

Also drop the stray per-icon Cur_Anim unload, and call wl_free_ship_class_data before loading in wl_init_ship_class_data.

In draft as it depends on #7737.

[JohnAFernandez]

Nothing really sticks out to me. My attempt at this was much more invasive, but this PR is pretty much as simple as it could be.

[Goober5000]

It definitely helped to do the data structure redesign first. Thanks!