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>

selfhost/backend/emit_decl.ludic

@@ -139,6 +139,7 @@ function emit_program() -> void {
   g_uses_strslice = false
   g_uses_loopback = false
   g_uses_expect = false
+  g_uses_panic = false
   loc_name = new []pointer;  loc_reg = new []pointer;  loc_ty = new []pointer;  loc_mut = new []int
   brk_lbl = new []pointer;   cnt_lbl = new []pointer
   self_stk = new []pointer
@@ -184,6 +185,10 @@ function emit_program() -> void {
   if g_uses_unicodert { emit_unicode_prelude() } # @fn_uni_len/valid/decode/case/truncate/grapheme (UTF-8)
   if g_uses_fsrt { emit_fs_prelude() }           # @fn_fs_*/fn_path_*/fn_mime_* (libc + string ops)
   if g_uses_datert { emit_datetime_prelude() }   # @fn_days_from_civil / @fn_civil_from_days conversions
+  if g_uses_panic {                               # panic/assert: located abort to stderr
+    emith("declare i32 @fprintf(ptr, ptr, ...)\n")
+    emith("@.fmt_panic = private unnamed_addr constant [6 x i8] c\"%s%s\\0A\\00\"\n")
+  }
 }
 
 # Flush the emitted IR. With a null path it goes to stdout (the pipe the shell