Avatar loader & customization system: merge onto main + fix regressions, add smoke coverage

Merges the pre-existing avatar system implementation (catalog, slot
registry, loadout, glTF-based loader, customization UI, visualizer --
~2300 lines across scripts/avatar/) onto current main, which had
diverged significantly since this branch was last touched (PC1 UI
fixes, PC2's playable level/player/multiplayer work).

Fixes found while integrating:
- project.godot had wholesale-replaced the [autoload] section again
  (same recurring pattern as the multiplayer server/client branches),
  dropping the ApiClient autoload entirely. Restored it additively
  alongside AvatarSystem.
- All other real conflicts (main_menu.tscn/.gd, main.tscn, .godot
  cache) were pure "this branch re-created a stale duplicate of
  something main already has" -- resolved by taking main's version,
  confirmed via `git diff` that this task's own changes never touched
  those files' actual logic.

New: scripts/smoke_test_avatar_system.gd -- confirms AvatarSystem is a
real autoload, the committed catalog.json parses, loadout equip/
validate/JSON-round-trip works against real catalog entries, and --
since assets/avatars/*.glb model files don't exist yet -- that
AvatarLoader.load_base_body() fails gracefully (null + load_error
signal) rather than crashing when given a real catalog entry with no
backing file. 10/10 passing.

Verified no regressions: all 10 pre-existing Godot smoke suites still
pass against a live backend, and all 61 backend pytest tests pass.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01M6rUEynmv2QmweocAQYARN

scripts/smoke_test_avatar_system.gd

@@ -0,0 +1,98 @@
+extends SceneTree
+## Smoke test for the avatar catalog/slot/loadout/system layers.
+##
+## Real .glb model assets don't exist yet (assets/avatars/catalog.json
+## references them, the files themselves are a tracked user to-do -- see
+## the "AGP MVP status" project memory). This test covers everything that
+## doesn't require them (catalog parsing, slot registry, loadout equip/
+## validate, the AvatarSystem orchestrator's autoload wiring) and confirms
+## the one thing that DOES touch missing files -- AvatarLoader.load_base_body()
+## -- fails gracefully (emits load_error, returns null) instead of crashing.
+## Visual rendering itself can't be verified until real assets exist.
+##
+## Run with:
+##     godot --headless --path . --script res://scripts/smoke_test_avatar_system.gd
+
+var passed := 0
+var failed := 0
+
+
+func _initialize() -> void:
+	_run()
+
+
+func _run() -> void:
+	await process_frame
+
+	# 1. AvatarSystem is a real autoload, not just a loose script.
+	var system := root.get_node_or_null("AvatarSystem")
+	if not _check("AvatarSystem autoload exists", system != null):
+		_finish()
+		return
+
+	# 2. The real catalog.json (committed to assets/avatars/) actually parses.
+	var err: Error = system.load_catalog()
+	_check("catalog loads without error", err == OK)
+	_check("AvatarSystem reports catalog loaded", system.is_catalog_loaded)
+
+	var base_bodies: Array[String] = system.get_assets_for_slot("base_body")
+	_check("catalog has at least one base_body entry", base_bodies.size() > 0)
+
+	# 3. Loadout: create, equip, validate against the real catalog.
+	# base_body is its own dedicated field on AvatarLoadout, not something
+	# set via the generic per-slot set_equipment() -- AvatarSlotRegistry's
+	# SLOT_IDS deliberately excludes "base_body" (it's the foundational
+	# model you start from, not a layered attachment slot).
+	var loadout := AvatarLoadout.new()
+	if base_bodies.size() > 0:
+		loadout.base_body = base_bodies[0]
+	_check("loadout has base_body set",
+		base_bodies.size() == 0 or not loadout.base_body.is_empty())
+
+	var hair_options: Array[String] = system.get_assets_for_slot("hair")
+	if hair_options.size() > 0:
+		var equip_ok: bool = loadout.add_accessory(hair_options[0])
+		_check("adding a real catalog accessory succeeds", equip_ok)
+
+	# 4. Loadout serializes and survives a round trip.
+	var json_str: String = loadout.to_json()
+	_check("loadout serializes to non-empty JSON", json_str.length() > 0)
+	var restored: AvatarLoadout = system.load_loadout_from_json(json_str)
+	_check("loadout round-trips through JSON", restored != null and restored.is_valid())
+
+	# 5. The part that DOES need real files: confirm it fails gracefully
+	# rather than crashing, since assets/avatars/*.glb genuinely don't
+	# exist yet. A crash here would be a real bug; a clean load_error
+	# signal + null return is the correct, already-designed-for behavior.
+	if base_bodies.size() > 0:
+		# A plain bool captured by a lambda is captured BY VALUE in
+		# GDScript -- mutating it inside the closure doesn't propagate to
+		# this outer scope. A single-element Array is the standard
+		# workaround (confirmed the hard way earlier in this same session,
+		# see scripts/smoke_test_network_client.gd's own comment on this).
+		var error_state := [false]
+		var error_handler := func(_msg): error_state[0] = true
+		system.loader.load_error.connect(error_handler)
+		var result: Node3D = system.loader.load_base_body(base_bodies[0])
+		system.loader.load_error.disconnect(error_handler)
+		_check("load_base_body against a missing .glb returns null (not a crash)", result == null)
+		_check("load_base_body against a missing .glb emits load_error (not silent)", error_state[0])
+	else:
+		print("  SKIP: no base_body entries in catalog to test the failure path against")
+
+	_finish()
+
+
+func _check(description: String, condition: bool) -> bool:
+	if condition:
+		print("  PASS: ", description)
+		passed += 1
+	else:
+		printerr("  FAIL: ", description)
+		failed += 1
+	return condition
+
+
+func _finish() -> void:
+	print("\n=== SMOKE RESULT: %d/%d passed ===" % [passed, passed + failed])
+	quit(0 if failed == 0 else 1)