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/print.zig

@@ -145,6 +145,7 @@ fn printInst(instruction: *const Inst, ref_idx: u32, tt: *const TypeTable, write
         .const_undef => try writer.writeAll("const undef : "),
         .is_comptime => try writer.writeAll("is_comptime : "),
         .interp_print_frames => try writer.writeAll("interp_print_frames : "),
+        .trace_frame => try writer.writeAll("trace_frame : "),
         .const_type => |tid| try writer.print("const type({s}) : ", .{tt.typeName(tid)}),
 
         // ── Arithmetic ──────────────────────────────────────────