refactor(client): decompose main.gd and game_state.gd god objects

Extract SnapshotHandler (static snapshot parsing), SnapshotConsumers
(non-dialogue consumers + audio handlers), and DialogueCoordinator
(dialogue consumers + signal handlers) as class_name scripts.

main.gd: 28KB → 13KB. game_state.gd: 20KB → 8.5KB.
Autoload parse-order safety maintained via load() inline pattern.

Ticket: #775

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

client/tests/test_main_scene.gd

@@ -1,5 +1,6 @@
 ## Smoke tests for main scene initialization
-## Run with: godot4 --headless --path . -s res://addons/gdUnit4/bin/GdUnitCmdTool.gd --ignoreHeadlessMode -a res://tests/
+## Run with: godot4 --headless --path . -s res://addons/gdUnit4/bin/GdUnitCmdTool.gd
+##           --ignoreHeadlessMode -a res://tests/
 class_name TestMainScene
 extends GdUnitTestSuite