game_data: Squadron loader — mission flight groups

Decode the per-stage Enumerate_Squadrons tables (UnitGroup_S<NN>.tbl)
into flight groups: formation shape, AI behaviour, side, and the member
craft. Definitions are delimited by the FormationID key; members are the
first `count` UN_ craft after the header (anything past them belongs to
the next squadron or the stage roster — the naive "all UN_ in span"
over-collected). e.g. the player's Rhino flight = a 2-craft TCAF
squadron of Delta Sabers flying Formation_2_Rhino. Schema hash varies
per stage so we match on the table name. Squadron id is positional and
left None when the id list and definitions don't line up (honest, not
guessed). +1 test.

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

crates/sylpheed-formats/examples/squadrons.rs

@@ -0,0 +1,11 @@
+use sylpheed_formats::{game_data, 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 sq=game_data::load_squadrons(&pak);
+    println!("{} squadron definitions", sq.len());
+    for s in sq.iter().filter(|s|s.side.as_deref()==Some("TCAF")).take(6){
+        let mem:Vec<String>=s.members.iter().map(|m|m.trim_start_matches("UN_").chars().take(18).collect()).collect();
+        println!("  {:8} {:22} {:24} x{}  {:?}", s.id.clone().unwrap_or("?".into()), s.formation_id.clone().unwrap_or_default(), s.ai_id.clone().unwrap_or_default(), s.count.unwrap_or(0), mem);
+    }
+}