PC3 Phase 1.4-1.6: avatars wired into the game end to end

- api_client.gd: get_avatar_loadout() / save_avatar_loadout()
- main_menu.gd: on successful auth (fresh or session-restore), load the
  catalog + fetch the saved loadout into AvatarSystem so the in-world
  player renders as the player's avatar. Best-effort: a slow/failed prime
  never gates getting in (UI goes authenticated first, prime runs after).
  New "Customize Avatar" button (enabled only when signed in).
- avatar_system.gd: local-player convenience API -- ensure_catalog(),
  set_loadout_from_dict() (from the backend's GET shape), has_loadout(),
  build_current_avatar().
- player.gd: swaps the placeholder capsule for the assembled avatar
  (feet-aligned to the capsule bottom) when a loadout is primed; silent
  fallback to the capsule otherwise. Camera stays first-person for now
  (third-person is a noted follow-up).
- avatar_visualizer.gd: rewrote it -- the old version referenced
  AvatarLoader.SlotResult / load_slot() which never existed, so it didn't
  compile and took avatar_customization_ui.gd down with it. Now a thin
  wrapper over AvatarLoader for the preview.
- avatar_customization_ui.gd: seeds from the live loadout, saves through to
  AvatarSystem + PUT /api/avatar/loadout (local file kept as fallback),
  fixed a Camera3D.look_at() before-in-tree crash.
- scenes/avatar_customization.tscn rebuilt (old one used the Godot-3
  `ViewportContainer` node type) + avatar_customization_screen.gd wrapper
  that returns to the menu on close.
- smoke_test_avatar_in_world.gd: primed loadout -> game.tscn -> player
  shows the assembled avatar, capsule hidden, rig + bone mounts present.

Verified: cold-cache smokes green (player 15/15, avatar_system 14/14,
avatar_in_world 10/10, main_menu_ui 13/13); against a live backend
main_menu_full_flow 12/12, main_menu_back 20/20, world_list 4/4,
enter_world 9/9, e2e 4/4; plus a manual register->default-loadout->
customize->save->re-fetch-persisted round trip.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015jtxAiu5dAW4WxCNdGkrkJ

scripts/avatar/avatar_system.gd

@@ -230,3 +230,46 @@ func get_assets_for_slot(slot_id: String) -> Array[String]:
 func get_asset_display_name(asset_id: String) -> String:
 	var asset := catalog.resolve(asset_id)
 	return asset.get("display_name", asset_id)
+
+
+# ---------------------------------------------------------------------------
+# Local-player convenience API
+#
+# The rest of the game (the main menu, the in-world player) only needs three
+# things: load the catalog once, hand us the player's loadout (as the dict the
+# backend returns), and ask for an assembled avatar node to drop into a scene.
+# ---------------------------------------------------------------------------
+
+## True once a loadout has been handed to us (from the backend or a default).
+func has_loadout() -> bool:
+	return current_loadout != null
+
+
+## Ensure the catalog is loaded (idempotent). Safe to await repeatedly.
+func ensure_catalog() -> bool:
+	if is_catalog_loaded:
+		return true
+	return load_catalog() == OK
+
+
+## Set the current player's loadout from the dict shape the backend's
+## GET /api/avatar/loadout returns (i.e. AvatarLoadout.to_json() parsed).
+## Returns true if it validated and was stored.
+func set_loadout_from_dict(loadout_dict: Dictionary) -> bool:
+	var lo := AvatarLoadout.from_json(JSON.stringify(loadout_dict))
+	if not lo.validate():
+		push_warning("AvatarSystem: rejecting invalid loadout dict: %s"
+			% str(lo.validation_errors))
+		return false
+	current_loadout = lo
+	loadout_changed.emit(current_loadout)
+	return true
+
+
+## Assemble a fresh avatar Node3D from the current loadout, ready to parent
+## into a scene (e.g. onto the in-world player). Returns null if there's no
+## loadout or the catalog isn't loaded or assembly failed.
+func build_current_avatar() -> Node3D:
+	if current_loadout == null or not is_catalog_loaded:
+		return null
+	return load_avatar(current_loadout)