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>

examples/library/errors.ludic

@@ -0,0 +1,28 @@
+# errors.ludic — error handling (issue #8): assert(cond, msg) guards an invariant
+# and aborts with a located message when it is broken; panic(msg) aborts outright.
+# This is the SUCCESS path — every assertion here holds, so nothing aborts and the
+# program runs to the end, printing:
+#   5 10 0 7 1
+# The failure path (a panic exits non-zero with `file:line: panic: …` on stderr)
+# is exercised separately in the test runner.
+program Errors {
+  function checked_div(a: int, b: int) -> int {
+    assert(b != 0, "division by zero")
+    return a / b
+  }
+
+  function clampi(v: int, lo: int, hi: int) -> int {
+    assert(lo <= hi, "lo must not exceed hi")
+    if v < lo { return lo }
+    if v > hi { return hi }
+    return v
+  }
+
+  entry {
+    print(checked_div(10, 2))    # 5
+    print(clampi(15, 0, 10))     # 10  (above hi)
+    print(clampi(0 - 3, 0, 10))  # 0   (below lo)
+    print(clampi(7, 0, 10))      # 7   (in range)
+    print(1)                     # reached: every assertion held
+  }
+}