viewer,ship: external parts ON by default + Ships catalog = composite ships only

Fixes the two things the user saw in the Ships browser:
- "Can't find the thrusters": show_external defaulted OFF, hiding the engine
  rig / bridge / turrets entirely. Externals are EXACT since the node-TRS fix,
  so the toggle now defaults ON (relabelled; off = bare hull bodies).
- "Some parts very far away": families without a composite scene graph (fighter
  morph sets like f002 whose detached parts are authored far from the origin,
  shared turret/prop part families) fell into the stack-at-origin fallback and
  rendered as piles with stray pieces. ShipModel now carries has_composite and
  the catalog lists only real assembled ships.

Offline audit across ALL stages (ship_audit example): the only far-away
outliers were composite-less f002 morph parts (now filtered); every capital
ship assembles coherently (e108 places its 7 turret instances, f105 its
mirrored shield generators). All primary composites are single-key tracks —
multikey exists only in animation composites (e901 attacks, missile-open),
which assemble_ship does not select.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

crates/sylpheed-formats/src/ship.rs

@@ -74,6 +74,10 @@ pub struct ShipModel {
     pub faction: Faction,
     /// Base geometry resource names to draw together, in directory order.
     pub parts: Vec<String>,
+    /// Whether a composite scene graph (`e_rou_<id>`) exists — i.e. this is a
+    /// real assembled ship. Families without one (fighter morph sets, shared
+    /// turret/prop parts) have no placement data and would stack at the origin.
+    pub has_composite: bool,
 }
 
 /// The `[a-z]NNN` id prefix of a resource name, if it has one (`e106_bdy_01`
@@ -146,7 +150,13 @@ pub fn ships_in_container(bytes: &[u8]) -> Vec<ShipModel> {
         let id = ship_id_of(n).unwrap().to_string();
         let entry = ships.entry(id.clone()).or_insert_with(|| {
             ids.push(id.clone());
-            ShipModel { id: id.clone(), faction: Faction::from_id(&id), parts: Vec::new() }
+            let has_composite = !composites_for(&names, &id).is_empty();
+            ShipModel {
+                id: id.clone(),
+                faction: Faction::from_id(&id),
+                parts: Vec::new(),
+                has_composite,
+            }
         });
         entry.parts.push(n.clone());
     }