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.

src/errors.zig

@@ -33,6 +33,18 @@ pub const SourceLoc = struct {
     }
 };
 
+/// The whole source line containing `byte_offset` (no trailing newline). Empty
+/// when `source` is empty. Used to embed the offending line in a trace `Frame`.
+pub fn lineAt(source: []const u8, byte_offset: u32) []const u8 {
+    if (source.len == 0) return source;
+    const at = @min(byte_offset, @as(u32, @intCast(source.len)));
+    var start: usize = at;
+    while (start > 0 and source[start - 1] != '\n') start -= 1;
+    var end: usize = at;
+    while (end < source.len and source[end] != '\n') end += 1;
+    return source[start..end];
+}
+
 pub const LineInfo = struct {
     line_num: u32,
     text: []const u8,