Add docs/SDL12.md from Claro v1.17.26 zip

docs/SDL12.md

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+# SDL 1.2 Support (Optional)
+
+Claro can be built with optional SDL 1.2 support for simple 2D graphics and input.
+
+## Build (MinGW example)
+
+Install SDL 1.2 development files, then compile with:
+
+```sh
+gcc -DCLARO_WITH_SDL12=1 -I<SDL_INCLUDE_DIR> src/claro.c -L<SDL_LIB_DIR> -lSDL -mwindows -O2 -o claro.exe
+```
+
+On some setups you may need `-lSDLmain` too.
+
+## Commands
+
+All SDL commands are grouped under `SDL ...`:
+
+- `SDL OPEN <w> <h> <title>`
+- `SDL CLEAR <r> <g> <b>`
+- `SDL RECT <x> <y> <w> <h> <r> <g> <b>` (filled rectangle)
+- `SDL PRESENT`
+- `SDL POLL` (updates variables `SDLEVENT` and `SDLKEY`)
+- `SDL DELAY <ms>`
+- `SDL CLOSE`
+
+### Event variables
+
+After `SDL POLL`, Claro variables update:
+
+- `SDLEVENT` = `"NONE" | "QUIT" | "KEYDOWN" | "KEYUP" | "OTHER"`
+- `SDLKEY` = key code number (SDLKey) for KEYDOWN/KEYUP, otherwise `0`
+
+## Sprite / bitmap support
+
+- `SDL LOADBMP <path> AS var` → loads a BMP and stores a numeric handle in `var`
+- `SDL BLIT <handle> AT x y` → draws the loaded bitmap at x,y
+- `SDL KEYDOWN <keycode> AS var` → sets var to 1/0 based on current key state
+
+- `SDL BLITCLIP <handle> SRC sx sy sw sh AT x y` → blit part of a surface (sprite-sheet)
+- `SDL MOUSE AS mx my mb` → writes mouse x/y and button bitmask