fix: every external-scanner grammar crashed pardes on parse (rust/cpp/python/...; e.g. opening agave cpi.rs or any tracy .cpp), Debug AND ReleaseSafe. Root cause: clang -fsanitize=function (in zig's default C UBSan set) traps at the runtime's indirect call of the scanner because grammars declare external_scanner_create() with EMPTY PARENS — a K&R non-prototype whose type hash differs from the void*(*)(void) pointer type. The ud1 trap lands on a bogus inlined line (stack.c:746), which cost the diagnosis a detour through rr (its gdbserver dies replaying past the task exit — core dump + coredumpctl worked; ud1 0x6(%eax) = SanitizerHandler kind 6 = function_type_mismatch; scanner-less c/zig grammars never crashed). Fix per review direction: -fno-sanitize=function on the grammar TUs in build.zig — uninstrumented callees make the runtime's call-site checks skip; the rest of UBSan stays live. Second half: fatal signals (SIGILL/SEGV/BUS/FPE) never run defers and bypassed the panic hook, leaving the terminal raw after a crash — root.debug.handleSegfault override now runs vaxis.recover() before std.debug.defaultHandleSegfault, verified in a raw pty (kill -ILL $PPID: rmcup + mouse resets precede the trace). Verified: rust/cpp/python opens work with real highlighting (snapstyle: keywords/strings/comments colored), agave cpi.js 2.7k-line open fine, suite 30/30, ReleaseSafe build opens rust identically.

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