postfx analític: nou shader + estructures chroma_min/max + scan_*

- Substitueix postfx.frag per la versió analítica amb smoothstep
- PostFXUniforms 12→16 floats (64B, 4×vec4): afegeix chroma_min/max,
  scan_dark_ratio, scan_dark_floor, scan_edge_soft
- PostFXParams i PostFXPreset adopten els nous camps amb defaults d'AEE
- MSL extret a source/core/rendering/sdl3gpu/msl/{postfx_vert,postfx_frag,
  crtpi_frag}.msl.h (estil Rendering::Msl::kXxx)
- SPIR-V regenerat (postfx_frag_spv.h: 13648 bytes)
- options.cpp llegeix 'chroma' antic com compat (assigna a min i max);
  escriu els 6 presets per defecte (CRT/NTSC/CURVED/SCANLINES/SUBTLE/CRT LIVE)
  amb els valors d'aee_arcade

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CMakeLists.txt

@@ -169,28 +169,20 @@ if(NOT APPLE AND NOT EMSCRIPTEN)
 
     set(SHADER_POSTFX_VERT_SRC  "${SHADERS_DIR}/postfx.vert")
     set(SHADER_POSTFX_FRAG_SRC  "${SHADERS_DIR}/postfx.frag")
-    set(SHADER_UPSCALE_FRAG_SRC "${SHADERS_DIR}/upscale.frag")
-    set(SHADER_DOWNSCALE_FRAG_SRC "${SHADERS_DIR}/downscale.frag")
     set(SHADER_CRTPI_FRAG_SRC   "${SHADERS_DIR}/crtpi_frag.glsl")
 
     set(SHADER_POSTFX_VERT_H    "${HEADERS_DIR}/postfx_vert_spv.h")
     set(SHADER_POSTFX_FRAG_H    "${HEADERS_DIR}/postfx_frag_spv.h")
-    set(SHADER_UPSCALE_FRAG_H   "${HEADERS_DIR}/upscale_frag_spv.h")
-    set(SHADER_DOWNSCALE_FRAG_H "${HEADERS_DIR}/downscale_frag_spv.h")
     set(SHADER_CRTPI_FRAG_H     "${HEADERS_DIR}/crtpi_frag_spv.h")
 
     set(ALL_SHADER_HEADERS
         "${SHADER_POSTFX_VERT_H}"
         "${SHADER_POSTFX_FRAG_H}"
-        "${SHADER_UPSCALE_FRAG_H}"
-        "${SHADER_DOWNSCALE_FRAG_H}"
         "${SHADER_CRTPI_FRAG_H}"
     )
     set(ALL_SHADER_SOURCES
         "${SHADER_POSTFX_VERT_SRC}"
         "${SHADER_POSTFX_FRAG_SRC}"
-        "${SHADER_UPSCALE_FRAG_SRC}"
-        "${SHADER_DOWNSCALE_FRAG_SRC}"
         "${SHADER_CRTPI_FRAG_SRC}"
     )