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>

source/gpu/gpu_shader_preset.hpp

@@ -0,0 +1,94 @@
+#pragma once
+
+#include <SDL3/SDL_gpu.h>
+#include <memory>
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+#include "gpu_texture.hpp"
+
+// ============================================================================
+// NTSCParams — uniform buffer for NTSC shader passes (set=3, binding=0)
+// Matches the layout in pass0_encode.frag and pass1_decode.frag.
+// Pushed via SDL_PushGPUFragmentUniformData(cmd, 0, &ntsc, sizeof(NTSCParams)).
+// ============================================================================
+struct 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 _pad[2];
+};
+static_assert(sizeof(NTSCParams) == 48, "NTSCParams must be 48 bytes");
+
+// ============================================================================
+// ShaderPass — one render pass in a multi-pass shader preset
+// ============================================================================
+struct ShaderPass {
+    SDL_GPUGraphicsPipeline* pipeline = nullptr;
+    GpuTexture*              target   = nullptr;  // null = swapchain (last pass)
+};
+
+// ============================================================================
+// GpuShaderPreset — loads and owns a multi-pass shader preset from disk.
+//
+// Directory layout:
+//   <dir>/preset.ini        — descriptor
+//   <dir>/pass0_xxx.vert    — GLSL 4.50 vertex shader source
+//   <dir>/pass0_xxx.frag    — GLSL 4.50 fragment shader source
+//   <dir>/pass0_xxx.vert.spv — compiled SPIRV (by CMake/glslc at build time)
+//   <dir>/pass0_xxx.frag.spv — compiled SPIRV
+//   ...
+// ============================================================================
+class GpuShaderPreset {
+public:
+    // Load preset from directory. swapchain_fmt is the target format for the
+    // last pass; intermediate passes use R16G16B16A16_FLOAT.
+    bool load(SDL_GPUDevice* device,
+              const std::string& dir,
+              SDL_GPUTextureFormat swapchain_fmt,
+              int w, int h);
+
+    void destroy(SDL_GPUDevice* device);
+
+    // Recreate intermediate render targets on resolution change.
+    void recreateTargets(SDL_GPUDevice* device, int w, int h);
+
+    int         passCount() const { return static_cast<int>(passes_.size()); }
+    ShaderPass& pass(int i)       { return passes_[i]; }
+
+    const std::string& name() const { return name_; }
+
+    // Read a float parameter parsed from preset.ini (returns default_val if absent).
+    float param(const std::string& key, float default_val) const;
+
+private:
+    std::vector<ShaderPass>                passes_;
+    std::vector<std::unique_ptr<GpuTexture>> targets_;  // intermediate render targets
+    std::string                            name_;
+    std::string                            dir_;
+    std::unordered_map<std::string, float> params_;
+    SDL_GPUTextureFormat                   swapchain_fmt_ = SDL_GPU_TEXTUREFORMAT_R8G8B8A8_UNORM;
+
+    // Entries read from preset.ini for each pass
+    struct PassDesc {
+        std::string vert_name;  // e.g. "pass0_encode.vert"
+        std::string frag_name;  // e.g. "pass0_encode.frag"
+    };
+    std::vector<PassDesc> descs_;
+
+    bool parseIni(const std::string& ini_path);
+
+    // Build a full-screen-triangle pipeline from two on-disk SPV files.
+    SDL_GPUGraphicsPipeline* buildPassPipeline(SDL_GPUDevice* device,
+                                               const std::string& vert_spv_path,
+                                               const std::string& frag_spv_path,
+                                               SDL_GPUTextureFormat target_fmt);
+};