ERR/E3.0 (slice 3a): embedded Frame trace resolution

Return-trace frames now resolve to real `func at file:line:col`
in-process — no DWARF, no symbolizer.

- New niladic, span-stamped `.trace_frame` IR op (mirrors is_comptime):
  carries no operands; each backend derives the frame from context.
  lower.zig's placeholderTraceFrame emits it; the existing
  sx_trace_push call consumes it.
- emit_llvm: resolve the op's span + current function to
  {file(basename), line, col, func}, build an interned Frame global
  ({string,i32,i32,string}, strings cached by content), push its
  address (ptrtoint).
- interp: pack (func_id << 32 | span.start) for the comptime resolver
  (slice 3b); never a pointer.
- sx_trace.c report_unhandled derefs SxFrame; trace.sx gains the Frame
  struct, frame_at -> *Frame, and field-reading to_string. Layout
  mirrored in 3 places with cross-ref comments.

Verified JIT + AOT. Snapshots 243/244/247 regenerated (placeholder ->
func at file:line:col). Gates: zig build, zig build test,
run_examples.sh -> 290 passed.

src/ir/interp.zig

@@ -640,6 +640,18 @@ pub const Interpreter = struct {
                 self.printInterpFrames();
                 return .{ .value = .void_val };
             },
+            .trace_frame => {
+                // Comptime frame: pack (func_id, span.start) so the slice-3b
+                // resolver can recover file:line:col via the IR/source tables.
+                // The interp never produces a `Frame*` — only the compiled
+                // backend does — so this stays a packed id, never a pointer.
+                const fid: u64 = if (self.call_chain.items.len > 0)
+                    self.call_chain.items[self.call_chain.items.len - 1].index()
+                else
+                    0;
+                const packed_frame: u64 = (fid << 32) | @as(u64, instruction.span.start);
+                return .{ .value = .{ .int = @bitCast(packed_frame) } };
+            },
             .const_type => |tid| return .{ .value = .{ .type_tag = tid } },
 
             // ── Arithmetic ──────────────────────────────────────