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>

docs/language/annotations/annot-clearcolor.md

@@ -0,0 +1,23 @@
+---
+id: annot-clearcolor
+name: "@ClearColor"
+category: annotations
+kind: annotation
+tokens: "@ClearColor"
+sig: "@ClearColor(0xRRGGBB) handler Draw phase Render { … }"
+tip: Declare a clear colour so the Render phase auto-clears + auto-presents for you.
+order: 62
+---
+
+<code>@ClearColor(N)</code> declares the framebuffer clear colour (a <code>0x00RRGGBB</code> integer) so the engine owns the per-frame clear and present: at the top of the Render phase it clears the framebuffer to <code>N</code>, and after all Render handlers run it presents the frame — so a game's Render handler carries no <code>Screen.clear</code> / <code>Screen.show</code> boilerplate, and the colour is configured declaratively rather than written in the handler body. It is opt-in: a program with no <code>@ClearColor</code> is byte-for-byte identical (it clears and presents itself, or the light system owns the present). Place it on the program's Render handler (or any declaration — it is program-wide).
+
+```ludic
+program ClearColorDemo {
+  property Tag { n: int = 0 }
+  model P { Tag }
+  @ClearColor(0x102030)
+  handler Draw phase Render {
+    Screen.fill_rectangle(10, 10, 4, 4, 0xffffff)
+  }
+}
+```