feat(render): #86 @ClearColor — Render phase auto-clears + auto-presents

@ClearColor(0xRRGGBB) declares the framebuffer clear colour, so the engine
owns the per-frame clear and flip: the Render phase clears to the colour at
the top and presents after the handlers run. Games drop the repeated
Screen.clear(color)/Screen.show() boilerplate, and the colour is configured
declaratively (an annotation) rather than in the handler body. Opt-in and
backward-compatible: a program with no @ClearColor is byte-for-byte identical
(it clears/presents itself, or the light system owns the present).

Parser reads @ClearColor(int) into g_clear_color/g_has_clear_color;
emit_game_main emits rt_clear before and rt_present after the Render phase,
gated on the flag. Example clear_color (pixel-readback verified — an undrawn
pixel holds the clear colour, proving the engine cleared), docs page +
inventory entry. Full suite 108/0, goldens byte-identical, fixpoint holds.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/docgen/inventory.json

@@ -24,7 +24,8 @@
     "annot-toclients",
     "annot-system",
     "annot-enginesystem",
-    "annot-namespace"
+    "annot-namespace",
+    "annot-clearcolor"
   ],
   "builtins": [
     "fn-print",
@@ -723,4 +724,4 @@
     "tiled-world_count",
     "tiled-world_map"
   ]
-}
+}