v1.6.3

1.6.3: parser/engine fixes + CI hardening

Engine: gfx_poke/gfx_peek consult screen_mode for the C64-style
bitmap/chars overlap at $3000-$37FF, so MEMSET 8192,8000,0 now
clears the whole bitmap plane.

Parser: IF cond THEN POKE a,b : stmt now runs the trailing stmt
(was swallowed by the non-gfx fallback). MOUSESET keyword-skip
off-by-one fixed. Mouse functions added to eval_factor allow-list.

New statement: BITMAPCLEAR clears the 8000-byte bitmap plane.

CI: gfx_video_test runs on every platform. 'make check' mirrors
CI locally; shared skip-list runner in tests/run_bas_suite.sh.

New example: examples/gfx_mouse_demo.bas + web/mouse-demo.html.