game_data: UnitRoster loader — per-mission battle compositions

Decode the EnumUnit tables (schema 35b8dc67) into mission rosters: the
distinct combatant `UN_*` ids for one battle, stage props (asteroids,
collision meshes) dropped. The tables aren't tagged with their stage on
disc, so `stage` is inferred from any `UN_S<NN>_…` prop id in the roster
and left None otherwise (honest — ~8 of 31 rosters self-identify).

Joined against load_units / load_vessels this gives each battle's
combatants with stats, e.g. S01 = ADAN Turret (100 HP), Attacker
(500 HP), Destroyer (10000 HP). Example `battle` prints them.

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

crates/sylpheed-formats/examples/battle.rs

@@ -0,0 +1,19 @@
+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_MAIN_GAME_E.pak")).unwrap();
+    let units=gd::load_units(&pak); let vessels=gd::load_vessels(&pak);
+    let hp=|id:&str|->String{
+        units.iter().find(|u|u.id.as_deref()==Some(id)).and_then(|u|u.hp).map(|h|format!("{h:.0}hp"))
+            .or_else(||vessels.iter().find(|v|v.id.as_deref()==Some(id)).and_then(|v|v.hp).map(|h|format!("{h:.0}HP")))
+            .unwrap_or("·".into())
+    };
+    let rosters=gd::load_unit_rosters(&pak);
+    for r in rosters.iter().filter(|r|r.stage.is_some()).take(4){
+        println!("\n{} — {} combatants:", r.stage.as_deref().unwrap(), r.units.len());
+        for u in r.units.iter().filter(|u|u.contains("ADAN")).take(6){
+            let short=u.trim_start_matches("UN_").split('_').skip(1).collect::<Vec<_>>().join("_");
+            println!("   {short:32} {}", hp(u));
+        }
+    }
+}