Add native WebAssembly backend

This introduces a lightweight WASM backend without SDL dependency,
achieving 60% binary size reduction (891K→357K) compared to SDL-based
approach.
- Add backend/wasm.c: Native WASM backend using Canvas 2D API directly
  * Direct Canvas rendering via EM_ASM (no SDL overhead)
  * Browser-native image decoding (JPEG/PNG)
  * Optimized for minimal runtime with MINIMAL_RUNTIME=1
  * Framebuffer accessor functions for zero-copy updates
- Remove twin_dispatch() from API and implementation
  * Eliminated unused function across all backends
  * All backends now use twin_dispatch_once() consistently
- Make SDL backend native-only
  * Removed Emscripten-specific code paths from backend/sdl.c
  * SDL now exclusively for native development/testing

Author: jserv

Makefile

@@ -16,11 +16,11 @@ ifneq ($(findstring emcc,$(CC_VERSION)),)
     CC_IS_EMCC := 1
 endif
 
-# Enforce SDL backend for Emscripten builds (skip during config targets)
+# Enforce compatible backend for Emscripten builds (skip during config targets)
 ifeq ($(filter $(check_goal),config defconfig),)
     ifeq ($(CC_IS_EMCC),1)
-        ifneq ($(CONFIG_BACKEND_SDL),y)
-            $(error Emscripten (WebAssembly) builds require SDL backend. Please run: env CC=emcc make defconfig)
+        ifneq ($(CONFIG_BACKEND_WASM),y)
+            $(error Emscripten (WebAssembly) builds require WASM backend. SDL backend is native-only.)
         endif
     endif
 endif
@@ -155,15 +155,8 @@ BACKEND := none
 ifeq ($(CONFIG_BACKEND_SDL), y)
 BACKEND = sdl
 libtwin.a_files-y += backend/sdl.c
-# Emscripten uses ports system for SDL2
-ifneq ($(CC_IS_EMCC), 1)
 libtwin.a_cflags-y += $(shell sdl2-config --cflags)
 TARGET_LIBS += $(shell sdl2-config --libs)
-else
-# Emscripten SDL2 port - flags needed for both compile and link
-libtwin.a_cflags-y += -sUSE_SDL=2
-TARGET_LIBS += -sUSE_SDL=2
-endif
 endif
 
 ifeq ($(CONFIG_BACKEND_FBDEV), y)
@@ -186,6 +179,12 @@ BACKEND = headless
 libtwin.a_files-y += backend/headless.c
 endif
 
+ifeq ($(CONFIG_BACKEND_WASM), y)
+BACKEND = wasm
+libtwin.a_files-y += backend/wasm.c
+# WASM backend uses Emscripten directly, no external libraries needed
+endif
+
 # Performance tester
 ifeq ($(CONFIG_PERF_TEST), y)
 target-$(CONFIG_PERF_TEST) += mado-perf
@@ -212,22 +211,35 @@ demo-$(BACKEND)_ldflags-y := \
 
 # Emscripten-specific linker flags for WebAssembly builds
 ifeq ($(CC_IS_EMCC), 1)
+# Base Emscripten flags for all backends
 demo-$(BACKEND)_ldflags-y += \
 	-sINITIAL_MEMORY=33554432 \
 	-sALLOW_MEMORY_GROWTH=1 \
 	-sSTACK_SIZE=1048576 \
-	-sUSE_SDL=2 \
-	-sMINIMAL_RUNTIME=0 \
 	-sDYNAMIC_EXECUTION=0 \
 	-sASSERTIONS=0 \
 	-sEXPORTED_FUNCTIONS=_main,_malloc,_free \
 	-sEXPORTED_RUNTIME_METHODS=ccall,cwrap \
 	-sNO_EXIT_RUNTIME=1 \
-	-Oz \
-	--preload-file assets \
-	--exclude-file assets/web
+	-Oz
+
+# Note: MINIMAL_RUNTIME incompatible with --preload-file, so we skip asset preloading
+# Assets can be loaded via HTTP requests instead
+ifeq ($(CONFIG_BACKEND_WASM), y)
+demo-$(BACKEND)_cflags-y += -flto
+demo-$(BACKEND)_ldflags-y += \
+	-sMINIMAL_RUNTIME=1 \
+	-sMALLOC=emmalloc \
+	-sFILESYSTEM=0 \
+	-sDISABLE_EXCEPTION_CATCHING=1 \
+	-sEXPORT_ES6=0 \
+	-sMODULARIZE=0 \
+	-sENVIRONMENT=web \
+	-sSUPPORT_ERRNO=0 \
+	-flto
 endif
 endif
+endif  # CONFIG_DEMO_APPLICATIONS
 
 # Font editor tool
 # Tools should not be built for WebAssembly