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/editors/jetbrains/src/main/kotlin/io/ludic/ide/LudicTokens.kt

@@ -64,7 +64,7 @@ object LudicVocabulary {
         "sprites_load", "draw_sprite", "draw_sprite_scaled", "ui_build", "ui_open",
         "ui_tick", "ui_render", "ui_clicked", "ui_set_text", "ui_set_int", "ui_focus",
         "ui_focused", "ui_visible", "key", "reg", "set_reg", "self", "save", "load",
-        "status", "print", "string", "quit",
+        "status", "print", "string", "quit", "panic", "assert",
         // compiler intrinsics: the floor the Ludic-written runtime stands on
         "bytes", "words", "resize", "free", "fill",         "arg_count", "arg", "file_stderr", "file_stdout",
         "offset",