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-viewer/src/ui.rs

@@ -1590,7 +1590,7 @@ fn draw_ships_ui(
                 }
             });
             ui.horizontal(|ui| {
-                ui.checkbox(&mut ships.show_external, "Show external parts");
+                ui.checkbox(&mut ships.show_external, "External parts (bridge/engines/turrets)");
                 ui.label(
                     egui::RichText::new("(bridge / shield gens / engines — approximate placement)")
                         .weak()