feat(shaders): sistema de shaders runtime amb presets externs

- Afegir GpuShaderPreset i ShaderManager per carregar shaders des de data/shaders/
- Implementar preset ntsc-md-rainbows (2 passos: encode + decode MAME NTSC)
- Render loop multi-pass per shaders externs (targets intermedis R16G16B16A16_FLOAT)
- cycleShader(): cicla OFF→PostFX natius→shaders externs amb tecla X
- --shader <nom> per arrancar directament amb un preset extern
- CMake auto-descubreix i compila data/shaders/**/*.vert/.frag → .spv
- HUD F1 mostra 'Shader: <nom>' quan hi ha shader extern actiu

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

data/shaders/ntsc-md-rainbows/pass0_encode.frag

@@ -0,0 +1,69 @@
+#version 450
+// license:BSD-3-Clause
+// copyright-holders:Ryan Holtz,ImJezze
+// Adapted from mame_ntsc_encode.slang for SDL3 GPU / Vulkan SPIRV
+
+layout(location=0) in vec2 v_uv;
+layout(location=0) out vec4 FragColor;
+
+layout(set=2, binding=0) uniform sampler2D Source;
+
+layout(set=3, binding=0) uniform NTSCParams {
+    float source_width;
+    float source_height;
+    float a_value;
+    float b_value;
+    float cc_value;
+    float scan_time;
+    float notch_width;
+    float y_freq;
+    float i_freq;
+    float q_freq;
+    float _pad0;
+    float _pad1;
+} u;
+
+const float PI  = 3.1415927;
+const float PI2 = PI * 2.0;
+
+void main() {
+    vec2 source_dims = vec2(u.source_width, u.source_height);
+
+    // p_value=1: one texel step per sub-sample (no horizontal stretch)
+    vec2 PValueSourceTexel = vec2(1.0, 0.0) / source_dims;
+
+    vec2 C0 = v_uv + PValueSourceTexel * vec2(0.00, 0.0);
+    vec2 C1 = v_uv + PValueSourceTexel * vec2(0.25, 0.0);
+    vec2 C2 = v_uv + PValueSourceTexel * vec2(0.50, 0.0);
+    vec2 C3 = v_uv + PValueSourceTexel * vec2(0.75, 0.0);
+
+    vec4 Cx = vec4(C0.x, C1.x, C2.x, C3.x);
+    vec4 Cy = vec4(C0.y, C1.y, C2.y, C3.y);
+
+    vec4 Texel0 = texture(Source, C0);
+    vec4 Texel1 = texture(Source, C1);
+    vec4 Texel2 = texture(Source, C2);
+    vec4 Texel3 = texture(Source, C3);
+
+    vec4 HPosition = Cx;
+    vec4 VPosition = Cy;
+
+    const vec4 YDot = vec4(0.299,     0.587,    0.114,     0.0);
+    const vec4 IDot = vec4(0.595716, -0.274453, -0.321263,  0.0);
+    const vec4 QDot = vec4(0.211456, -0.522591,  0.311135,  0.0);
+
+    vec4 Y = vec4(dot(Texel0, YDot), dot(Texel1, YDot), dot(Texel2, YDot), dot(Texel3, YDot));
+    vec4 I = vec4(dot(Texel0, IDot), dot(Texel1, IDot), dot(Texel2, IDot), dot(Texel3, IDot));
+    vec4 Q = vec4(dot(Texel0, QDot), dot(Texel1, QDot), dot(Texel2, QDot), dot(Texel3, QDot));
+
+    float W    = PI2 * u.cc_value * u.scan_time;
+    float WoPI = W / PI;
+
+    float HOffset = u.a_value / WoPI;
+    float VScale  = u.b_value * source_dims.y / WoPI;
+
+    vec4 T  = HPosition + vec4(HOffset) + VPosition * vec4(VScale);
+    vec4 TW = T * W;
+
+    FragColor = Y + I * cos(TW) + Q * sin(TW);
+}