game_data: Arsenal loader — player weapon options per hardpoint

Decode the player weapon arsenal from the UNITS tables
(GP_HANGAR_ARSENAL.pak): the selectable weapons per Delta Saber
hardpoint. Each `STANDARD_<hp>` header (Nose / Arm1-3) lists its options
up to the next header — extracted by shape, unioned + deduped across
configs.

The arsenal reads with the game's own names: NOSE guns (Stiletto,
Rapier, Broad Sword, Machine Cannon), ARM missiles/bombs (Falcon, Eagle,
White Shark, Piranha, Tomahawk Rail Gun, Maelstrom Bomb). Example
`arsenal` prints them. +1 test.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/sylpheed-formats/examples/arsenal.rs

@@ -0,0 +1,9 @@
+use sylpheed_formats::{game_data as gd, PakArchive};
+fn main(){
+    let disc=std::env::var("SYLPHEED_DISC").unwrap();
+    let pak=PakArchive::open(format!("{disc}/dat/GP_HANGAR_ARSENAL.pak")).unwrap();
+    let a=gd::load_arsenal(&pak);
+    for (hp,list) in [("NOSE",&a.nose),("ARM1",&a.arm1),("ARM2",&a.arm2),("ARM3",&a.arm3)]{
+        println!("{hp} ({}): {:?}", list.len(), list.iter().take(8).collect::<Vec<_>>());
+    }
+}