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/emit_llvm.zig

@@ -4721,8 +4721,9 @@ pub const LLVMEmitter = struct {
             self.cached_i32, // line
             self.cached_i32, // col
             str_ty, // func
+            str_ty, // line_text (the source line, for the snippet)
         };
-        self.frame_struct_type = c.LLVMStructTypeInContext(self.context, &field_types, 4, 0);
+        self.frame_struct_type = c.LLVMStructTypeInContext(self.context, &field_types, 5, 0);
         return self.frame_struct_type.?;
     }
 
@@ -4761,9 +4762,10 @@ pub const LLVMEmitter = struct {
             c.LLVMConstInt(self.cached_i32, loc.line, 0),
             c.LLVMConstInt(self.cached_i32, loc.col, 0),
             self.buildStringConst(func_name),
+            self.buildStringConst(errors.lineAt(src, instruction.span.start)),
         };
         const frame_ty = self.getFrameStructType();
-        const frame_const = c.LLVMConstNamedStruct(frame_ty, &fields, 4);
+        const frame_const = c.LLVMConstNamedStruct(frame_ty, &fields, 5);
         const g = c.LLVMAddGlobal(self.llvm_module, frame_ty, "trace.frame");
         c.LLVMSetInitializer(g, frame_const);
         c.LLVMSetGlobalConstant(g, 1);