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/ir/interp.zig

@@ -677,18 +677,21 @@ pub const Interpreter = struct {
                 const file = std.fs.path.basename(file_full);
                 var line: i64 = 1;
                 var col: i64 = 1;
+                var line_text: []const u8 = "";
                 if (self.source_map) |sm| {
                     if (sm.get(file_full)) |src| {
                         const loc = errors.SourceLoc.compute(src, offset);
                         line = @intCast(loc.line);
                         col = @intCast(loc.col);
+                        line_text = errors.lineAt(src, offset);
                     }
                 }
-                const fields = self.alloc.alloc(Value, 4) catch return .{ .value = .undef };
+                const fields = self.alloc.alloc(Value, 5) catch return .{ .value = .undef };
                 fields[0] = .{ .string = file };
                 fields[1] = .{ .int = line };
                 fields[2] = .{ .int = col };
                 fields[3] = .{ .string = func_name };
+                fields[4] = .{ .string = line_text };
                 return .{ .value = .{ .aggregate = fields } };
             },
             .const_type => |tid| return .{ .value = .{ .type_tag = tid } },