feat(errors): panic(msg) + assert(cond, msg) with file:line — no raw crashes (#8)

RFC decision (the split the issue recommended): programmer bugs abort loud and
located; recoverable failures become values. This ships the first half.

panic(msg) prints `file:line: panic: <msg>` to stderr and aborts the process with
exit code 1 — a clear, located error instead of a segfault or a silent wrong
result. assert(cond, msg) is the guarded form: it aborts with `file:line:
assertion failed: <msg>` only when cond is false, otherwise execution continues.
The location is baked in at compile time (the call node carries its source line,
g_src_name carries the file); the message is any string.

Both lower in emit_call to an fprintf-to-stderr + exit(1) + unreachable tail
(assert branches on the condition first). @fprintf and the format constant are
declared on demand (g_uses_panic), so a program that never panics is unchanged —
and the compiler's own source uses neither, so the C-free bootstrap fixpoint holds.

- panic/assert registered as builtins across the vocabulary (ludic_syntax.h, the
  JetBrains lexer, the TextMate grammar) and documented (docs/language/builtins/)
- examples/library/errors.ludic covers the success path (asserts hold, program
  runs to the end); a panic_case in the suite covers the failure path (non-zero
  exit + the located stderr message). x test is now 69 checks.

Deferred: recoverable failures as `try`/`else` values (needs the tagged-union
type system, #1) and a top-level `recover` for the dev game loop.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/ludic-tools/ludic_syntax.h

@@ -141,6 +141,8 @@ static const LBuiltin LUDIC_BUILTINS[] = {
  {"print","print(x)","Print a value (int or string) and a newline to stdout."},
  {"string","string(x) -> string","Convert an int/bool/fixed to text (a string passes through)."},
  {"quit","quit()","Stop the frame loop and exit."},
+ {"panic","panic(msg: string)","Print `file:line: panic: msg` to stderr and abort (exit 1) — a located error, not a crash."},
+ {"assert","assert(cond: bool, msg: string)","Abort with `file:line: assertion failed: msg` when cond is false — for programmer-bug invariants."},
  {0,0,0}
 };