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/inst.zig

@@ -96,6 +96,14 @@ pub const Op = union(enum) {
     /// code it's a no-op (only ever reached from a dead `is_comptime()` branch,
     /// where there is no interpreter stack to walk).
     interp_print_frames,
+    /// ERR E3.0 slice 3a — a return-trace frame value (`u64`) for the push site.
+    /// Niladic + span-stamped: it carries NO operands; each backend derives the
+    /// frame from its own context. `emit_llvm` resolves this instruction's span
+    /// + the current function → `{file,line,col,func}`, interns a `Frame` global,
+    /// and yields its address (`ptrtoint`). `interp` yields a packed
+    /// `(func_id << 32 | span.start)` for the comptime resolver (slice 3b). The
+    /// result feeds the existing `sx_trace_push(u64)` call.
+    trace_frame,
     /// Comptime-only Type value. Carried as a `Value.type_tag(TypeId)`
     /// in the interpreter. NEVER emitted to LLVM — types are erased
     /// after lowering. `emit_llvm` bails loudly if it sees one,