Add project skeleton: Godot setup, tooling, and smoke test

Pins Godot 4.6.3-stable and GUT 9.6.0 (9.6.1+ reference
AccessibilityServer, unavailable in 4.6.3) via tools/setup.sh. Wires
make check (gdlint + gdformat config for 4-space indent, headless
GUT run, and a JSON well-formedness check ahead of real schema
validation). Seven autoload stubs match the architecture in the
cahier des charges; real logic lands with the following steps.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitattributes

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+# Texte : fins de ligne normalisees en LF
+* text=auto eol=lf
+
+*.gd text eol=lf
+*.gdshader text eol=lf
+*.tscn text eol=lf
+*.tres text eol=lf
+*.cfg text eol=lf
+*.json text eol=lf
+*.md text eol=lf
+*.cs text eol=lf
+*.py text eol=lf
+*.sh text eol=lf
+Makefile text eol=lf
+
+# Assets : binaires, jamais de conversion de fin de ligne
+*.png binary
+*.jpg binary
+*.jpeg binary
+*.webp binary
+*.ogg binary
+*.wav binary
+*.mp3 binary
+*.ttf binary
+*.otf binary
+*.woff binary
+*.woff2 binary
+*.zip binary
+*.ico binary
+*.svg binary