Add Emscripten/WebAssembly build target

Builds a browser-playable .html/.js/.wasm/.data via `./build-web.sh` (no
new CMake top-level target — Emscripten is detected at configure time and
takes the existing kobodl through a web-flavored pipeline).

Key pieces:
- CMakeLists.txt: route SDL3 through `--use-port=sdl3`; substitute
  `--use-port=libpng` for the missing sdl3_image port (Emscripten 5.0.7
  ships only sdl3 + sdl3_ttf). pthread (atomics+bulk-memory) lifted to
  kobo_deps so every TU agrees with wasm-ld's --shared-memory. Link with
  -sASYNCIFY (avoids rewriting kobo's blocking main loop around
  emscripten_set_main_loop), -sALLOW_MEMORY_GROWTH/-sMAXIMUM_MEMORY=1gb,
  -sPTHREAD_POOL_SIZE=4, -lidbfs.js, and --preload-file data@/data.
- graphics/img_compat.{h,c}: libpng-backed IMG_Load shim, used in
  sprite.c / gfxengine.cpp / filters.c via #include "img_compat.h".
  Native builds passthrough to <SDL3_image/SDL_image.h> unchanged.
- web/shell.html: HTML wrapper that mounts /home as IDBFS for persistent
  scores/config. Startup gated on addRunDependency until syncfs(true)
  finishes, then /home/scores is mkdir'd post-load (the game's addPlayer
  doesn't auto-create the score dir; see TODO in score.cpp).
  beforeunload + visibilitychange flushes /home back to IndexedDB.
- build-web.sh: emcmake wrapper + a static server that sets the
  COOP/COEP headers SharedArrayBuffer/pthreads require.

Source fixes needed to build under Emscripten's libc/libcxx:
- Renamed enemy_kind `pipe2` -> `pipe2k` (collided with libc pipe2(2)
  always declared in Emscripten's <unistd.h>).
- graphics/toolkit.cpp: added missing <stdio.h> for snprintf (Emscripten's
  libcxx headers don't pull it in transitively).
- states.cpp: gated the "Quit Kobo Deluxe" main-menu button on
  !__EMSCRIPTEN__; exit() on the web leaves a dead canvas.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

graphics/CMakeLists.txt

@@ -9,6 +9,7 @@ add_library(graphics STATIC
     toolkit.cpp
     vidmodes.c
     region.c
+    img_compat.c
 )
 
 target_include_directories(graphics

graphics/filters.c

@@ -23,7 +23,7 @@
 #include <math.h>
 #include "logger.h"
 #include "sdl_compat.h"
-#include <SDL3_image/SDL_image.h>
+#include "img_compat.h"
 #include "sprite.h"
 #include "filters.h"
 

graphics/gfxengine.cpp

@@ -28,7 +28,7 @@
 
 #include "gfxengine.h"
 #include "filters.h"
-#include <SDL3_image/SDL_image.h>
+#include "img_compat.h"
 #include "sdl_compat.h"
 #include "sofont.h"
 #include "window.h"

graphics/img_compat.c

@@ -0,0 +1,92 @@
+/*
+ * img_compat.c — libpng-backed IMG_Load for the Emscripten build.
+ *
+ * Returns an RGBA8 SDL_Surface. Handles palette, grayscale, tRNS, and 16-bit
+ * inputs by normalizing to 8-bit RGBA in the read pipeline.
+ */
+#ifdef __EMSCRIPTEN__
+
+#include "img_compat.h"
+
+#include <png.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+SDL_Surface *IMG_Load(const char *file)
+{
+    FILE *fp = fopen(file, "rb");
+    if(!fp)
+        return NULL;
+
+    unsigned char hdr[8];
+    if(fread(hdr, 1, 8, fp) != 8 || png_sig_cmp(hdr, 0, 8))
+    {
+        fclose(fp);
+        return NULL;
+    }
+
+    png_structp png = png_create_read_struct(PNG_LIBPNG_VER_STRING,
+                                             NULL, NULL, NULL);
+    if(!png) { fclose(fp); return NULL; }
+    png_infop info = png_create_info_struct(png);
+    if(!info)
+    {
+        png_destroy_read_struct(&png, NULL, NULL);
+        fclose(fp);
+        return NULL;
+    }
+
+    SDL_Surface *surf = NULL;
+    png_bytep *rows = NULL;
+
+    if(setjmp(png_jmpbuf(png)))
+    {
+        if(rows) free(rows);
+        if(surf) SDL_DestroySurface(surf);
+        png_destroy_read_struct(&png, &info, NULL);
+        fclose(fp);
+        return NULL;
+    }
+
+    png_init_io(png, fp);
+    png_set_sig_bytes(png, 8);
+    png_read_info(png, info);
+
+    png_uint_32 w, h;
+    int depth, ctype;
+    png_get_IHDR(png, info, &w, &h, &depth, &ctype, NULL, NULL, NULL);
+
+    if(depth == 16) png_set_strip_16(png);
+    if(ctype == PNG_COLOR_TYPE_PALETTE) png_set_palette_to_rgb(png);
+    if(ctype == PNG_COLOR_TYPE_GRAY && depth < 8)
+        png_set_expand_gray_1_2_4_to_8(png);
+    if(png_get_valid(png, info, PNG_INFO_tRNS))
+        png_set_tRNS_to_alpha(png);
+    if(ctype == PNG_COLOR_TYPE_GRAY || ctype == PNG_COLOR_TYPE_GRAY_ALPHA)
+        png_set_gray_to_rgb(png);
+    if(ctype == PNG_COLOR_TYPE_RGB || ctype == PNG_COLOR_TYPE_GRAY
+            || ctype == PNG_COLOR_TYPE_PALETTE)
+        png_set_filler(png, 0xFF, PNG_FILLER_AFTER);
+
+    png_read_update_info(png, info);
+
+    surf = SDL_CreateSurface((int)w, (int)h, SDL_PIXELFORMAT_RGBA32);
+    if(!surf)
+        png_error(png, "SDL_CreateSurface failed");
+
+    rows = (png_bytep *)malloc(sizeof(png_bytep) * h);
+    if(!rows)
+        png_error(png, "row table alloc failed");
+    for(png_uint_32 y = 0; y < h; ++y)
+        rows[y] = (png_bytep)((unsigned char *)surf->pixels + y * surf->pitch);
+
+    png_read_image(png, rows);
+    png_read_end(png, NULL);
+
+    free(rows);
+    png_destroy_read_struct(&png, &info, NULL);
+    fclose(fp);
+    return surf;
+}
+
+#endif /* __EMSCRIPTEN__ */

graphics/img_compat.h

@@ -0,0 +1,32 @@
+/*
+ * img_compat.h — SDL3_image shim.
+ *
+ * On native builds, this is just a passthrough to <SDL3_image/SDL_image.h>.
+ * On Emscripten there is no SDL3_image port yet (only sdl3 and sdl3_ttf), so
+ * we provide an IMG_Load implementation backed by the libpng port. Only PNG
+ * decoding is needed — Kobo Deluxe's assets are all .png.
+ */
+#ifndef KOBO_IMG_COMPAT_H
+#define KOBO_IMG_COMPAT_H
+
+#ifdef __EMSCRIPTEN__
+
+#include <SDL3/SDL.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+SDL_Surface *IMG_Load(const char *file);
+
+#ifdef __cplusplus
+}
+#endif
+
+#else  /* native */
+
+#include <SDL3_image/SDL_image.h>
+
+#endif
+
+#endif /* KOBO_IMG_COMPAT_H */

graphics/sprite.c

@@ -30,7 +30,7 @@ TODO: tables as needed when loading banks.
 #include <string.h>
 #include "logger.h"
 #include "sdl_compat.h"
-#include <SDL3_image/SDL_image.h>
+#include "img_compat.h"
 #include "sprite.h"
 #include "filters.h"
 

graphics/toolkit.cpp

@@ -23,6 +23,7 @@
 
 #include <string.h>
 #include <stdlib.h>
+#include <stdio.h>
 
 #include "window.h"
 #include "toolkit.h"