feat: thinner cel outline + procedural model actually holds its weapon

Outline: the inverted-hull width was tuned when the look was heavier and
now reads as a thick marker stroke. Cut every call site to roughly a third
(characters 0.012 -> 0.004, level geometry 0.015 -> 0.005, Neon Alley's
size-scaled clamp 0.02..0.05 -> 0.007..0.018) plus the shader default, so
the ink line stays readable without fattening every silhouette.

Third-person weapon on HumanoidModel: the gun was parented to root_pivot at
a fixed (-0.15, 1.0, 0.4) — floating 40 cm off the chest while both arms
posed at it from a distance. Give the rig a real right-hand node at the end
of the forearm and hang the weapon there, so it travels with the arm. Its
aim is re-derived from the body each frame (hand_r_pivot's global basis is
copied from root_pivot), otherwise the barrel would swing around with the
arm animation. The hold pose is retuned to match: right elbow tucked at the
ribs with a level forearm, left arm crossing to the handguard.

_seat_weapon_in_hand() slides each weapon along its own barrel axis so a
plausible grip lands in the fist regardless of where the artist left the
model origin. It measures with MeshInstance3D.get_aabb(), not
mesh.get_aabb() — the FBX gun parts are skinned, so the Mesh resource still
carries bind-pose bounds tens of metres across.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

assets/shaders/toon_outline.gdshader

@@ -6,7 +6,7 @@ render_mode cull_front, unshaded;
 // culled, leaving a colored shell visible only at the silhouette.
 
 uniform vec4 outline_color : source_color = vec4(0.06, 0.05, 0.09, 1.0);
-uniform float outline_width : hint_range(0.0, 0.1) = 0.02;
+uniform float outline_width : hint_range(0.0, 0.1) = 0.006;
 
 void vertex() {
 	VERTEX += NORMAL * outline_width;