ERR/E3.0 (slice 3c): source snippet + caret in traces

Each trace frame now shows the offending source line with a `^` caret
under the column — in the catch-handler formatter, the failable-main C
reporter, and the comptime path.

The source line is embedded at compile time as a 5th Frame field
(line_text), not read from disk at runtime: the file field is a
basename and a runtime read would add a filesystem dependency that
fails under the test harness and on locked-down targets.

- errors.lineAt(src, offset): shared helper for the whole source line.
- Frame gains line_text (mirrored in emit_llvm getFrameStructType,
  trace.sx Frame, sx_trace.c SxFrame). emitTraceFrame embeds it; the
  interp .trace_resolve extracts it from the source map.
- trace.sx (new spaces helper) and the C reporter render the line +
  a col-aligned caret, guarded on a non-empty line_text.

Snapshots 243/244/247/253 regenerated. Gates: zig build, zig build
test, run_examples.sh -> 291 passed.

tests/expected/253-comptime-trace.txt

@@ -1,5 +1,9 @@
 comptime caught Bad
 error return trace (most recent call last):
   leaf at 253-comptime-trace.sx:15:5
+        raise error.Bad;
+        ^
   mid at 253-comptime-trace.sx:19:5
+        try leaf();
+        ^
 --- build done ---