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>

selfhost/backend/game/emit_world.ludic

@@ -486,7 +486,18 @@ function emit_game_main() -> void {
   emit_calls_for_phase("FixedUpdate")
   emit_calls_for_phase("Update")
   emit_calls_for_phase("LateUpdate")
+  # #86: @ClearColor(N) makes the Render phase auto-clear to N at the top and
+  # auto-present after the handlers run, so a game drops the clear/show boilerplate
+  # (the clear colour is declared, not written in the handler body). Opt-in — a game
+  # with no @ClearColor is byte-identical (it clears/shows itself, or the light
+  # system owns the present).
+  if g_has_clear_color and (find_fn("rt_clear") != null) {
+    emit("  call void @fn_rt_clear(i32 ");  emit(itoa(g_clear_color));  emit(")\n")
+  }
   emit_calls_for_phase("Render")
+  if g_has_clear_color and (find_fn("rt_present") != null) {
+    emit("  call void @fn_rt_present()\n")
+  }
   let fcur = emit_bind("load i32, ptr @L_frame")     # Time.frame(): count completed frames
   let fnext = emit_bind(`add i32 {fcur}, 1`)
   emit("  store i32 ");  emit(fnext);  emit(", ptr @L_frame\n")

selfhost/frontend/parse.ludic

@@ -443,6 +443,8 @@ var g_uses_tiled: bool = false    # Tiled.* -> splice the Tiled map runtime (#69
 var g_uses_reflect_io: bool = false  # Reflect.serialize/apply -> splice the reflection serializer
 var g_uses_esys: bool = false     # an engine-owned system component (SpriteAnim/Motion/Light2D) is declared -> splice systems.ludic + force the reflection ABI
 var g_uses_input: bool = false    # a program used Input.bind/down/poll/… (action maps + record/replay) -> splice input.ludic
+var g_has_clear_color: bool = false   # @ClearColor(N) declared -> the Render phase auto-clears + auto-presents (#86)
+var g_clear_color: int = 0            # the declared clear colour (0x00RRGGBB)
 var g_uses_anim_rt: bool = false  # Anim.play/clip/on_frame/fired or Motion.to (#48) -> splice systems.ludic + force the reflection ABI
 var g_uses_tween_rt: bool = false # Tween.to/chain/delay/… (#48) -> splice tween.ludic + run esys_tween each Update
 var g_uses_audio: bool = false    # Audio.* (#22) -> splice audio.ludic; a windowed build also links audio.ll + AVFoundation
@@ -542,8 +544,11 @@ function parse_one_decl() -> void {
     else { if (a == "System") { sys_phase = "Update"; if is_op("(") { pi = pi + 1; sys_phase = eat_id(); eat_op(")") } }   # @System(Phase) binary-module system (#64)
     else { if (a == "EngineSystem") { eat_op("("); esys_comp = eat_id(); eat_op(","); esys_phase = eat_id(); eat_op(")") }   # @EngineSystem(Comp, Phase) package engine system (#62)
     else { if (a == "Namespace") { eat_op("("); ns_name = eat_id(); eat_op(")") }   # @Namespace(Name) package Foo.* namespace (#62)
+    else { if (a == "ClearColor") { eat_op("(")                                     # @ClearColor(N) — Render auto-clear + auto-present (#86)
+        if toks[pi].kind != TK_INT { perr("@ClearColor expects an integer colour, e.g. @ClearColor(0x101018)") }
+        g_clear_color = toks[pi].ival;  g_has_clear_color = true;  pi = pi + 1;  eat_op(")") }
     else { if is_op("(") { var d = 0          # any other @anno(args) — parsed and skipped
-        while true { if is_op("(") { d = d + 1 };  if is_op(")") { d = d - 1 };  pi = pi + 1;  if d == 0 { break } } } } } } } } } } } } } } } } } } } } } } } }
+        while true { if is_op("(") { d = d + 1 };  if is_op(")") { d = d - 1 };  pi = pi + 1;  if d == 0 { break } } } } } } } } } } } } } } } } } } } } } } } } }
     skipnl()
   }
   if is_id("import") { pi = pi + 1
@@ -875,6 +880,8 @@ function parse_program() -> void {
   g_uses_reflect = false
   g_uses_esys = false
   g_uses_input = false
+  g_has_clear_color = false
+  g_clear_color = 0
   g_uses_light = false
   g_uses_value = false
   g_uses_xml = false