External weapon model cleanup: consolidate duplicated logic, fix several longstanding bugs

[Goober5000]

This is a cleanup pass over the external weapon model code ($Show Primary/Secondary Models:), consolidating duplicated logic and fixing the bugs that were hiding in the copies.

Consolidations

• Weapon page-in — weapons_page_in(), weapons_page_in_cheats(), and weapon_page_in() each duplicated the same ~60-line per-weapon block (model load, thruster flag, external model load, shockwaves, graphics paging). Extracted into weapon_page_in_one(); the cheats variant passes load_graphics = false to preserve its model-only behavior.
• Firing point selection — the "offset the shot by the external model's firing points" logic was copied in ship_fire_primary(), ship_fire_secondary(), and the AI line-of-sight check. Now shared as ship_get_external_model_fp_offset().
• Render slot placement — the per-slot math (angle-offset yaw, missile reload slide-back) was duplicated four times across ship_render_weapon_models() and HudGaugeHardpoints::render(). Now shared as ship_get_weapon_model_slot_transform().

Bug fixes

• "chain external model fps" never chained. All three copies of the firing point logic reset the chain counter against n_guns (the number of gun banks) instead of the number of firing points in bank 0. For the typical one-bank model the counter reset after every shot, so chaining never advanced; models with several banks could read past the end of the point array.
• The AI line-of-sight check advanced the chain counter, even though it's only a query, desyncing it from the shots actually fired.
• Gatling barrel-spin instances went stale after a mid-mission weapon swap. The lazily created model instance was guarded by a once-per-bank flag that nothing reset, so after set-primary-weapon, a script swap, or a rearm-precedence swap, the old weapon's instance leaked and the new weapon's barrels never spun. The instance is now keyed to the weapon it was created for and recreated on change (ship_get_external_weapon_model_instance()), which self-heals after every swap path.
• $Weapon Model Draw Distance: has been dead for years — nothing set MR_ATTACHED_MODEL since 1ef7688. The distance is now carried in model_render_params and enforced again. The old check also computed depth as if the weapon model's ship-local position were in world space; depth is now measured from the eye position in the parent's frame, which additionally fixes attached weapon models choosing their LOD from a garbage distance. MR_ATTACHED_MODEL is removed (nothing else referenced it).
• Hardpoints gauge misplaced launcher models — the external model launcher branch computed the view-rotated position but rendered at the untransformed local point.
• Never-firing assertion — the invalid-render-type assertion in the page-in code was a tautology inside its default case; it now asserts render_type == WRT_NONE.
• Empty-bank hardening — the render paths and ship_stop_fire_primary_bank() indexed Weapon_info[] without checking for an empty bank (-1), unlike other bank-iterating code.

Notes for modders

• Weapons with "chain external model fps" will now actually cycle through the external model's firing points; previously every shot came from point 0.
• $Weapon Model Draw Distance: (default 200) is enforced again, and attached weapon models now LOD correctly with distance — missions far from the world origin previously got worst-case LODs regardless of camera distance.

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