fix(ansi): u16 overflow on oversized CSI parameters

[kudasaixc]

The CSI parameter accumulator multiplied and added with no overflow protection. A sequence like ESC[99999m overflows the u16 parameter and, since the Zig objects are built with -O ReleaseSafe, trips the safety check and lands in the panic handler: the kernel silently freezes. Saturating arithmetic clamps oversized parameters to 65535 instead of bringing the machine down.

[kudasaixc]

PoC

The CSI parameter accumulator has no overflow guard:

val.params[val.param_index] = (val.params[val.param_index] * 10) + n;

params is [8]u16. A big numeric parameter overflows it, and since the Zig objects are built -O ReleaseSafe, the overflow hits a runtime safety check that calls the panic handler (cli; hlt) — a silent, unrecoverable freeze. It is reachable from any text routed through the terminal:

echo \x1b[99999m

99999 > 65535, so the * 10 overflows the u16 and the kernel locks up. Attacker-influenced output — echo, shell error messages echoing a bad command, and any future serial/pipe input — can hard-freeze the machine with a single escape sequence, i.e. a trivial denial of service on the console.

Fix uses saturating arithmetic (*|, +|) so oversized parameters clamp to 65535 instead of trapping.

[im-nymii]

LGTM !