ERR/E4.1: trace.print_interpreter_frames() — Phase E4 complete

The last E4 item: a comptime call-frame dump.

- New nullary `interp_print_frames` IR op (inst/print). The interpreter
  maintains a `call_chain` side-stack (push/pop a FuncId around each sx-bodied
  `call`, freed in deinit) and `printInterpFrames` appends the chain to its
  output — most-recent-last, with the dump frame itself skipped. emit_llvm
  makes the op a no-op: compiled code has no interpreter stack, and the only
  caller is `process.exit`'s dead `is_comptime()` branch.
- Lowered from a name-recognized `__interp_print_frames()` builtin
  (tryLowerReflectionCall + inferExprType → void).
- `trace.print_interpreter_frames()` wraps the builtin; wired into
  `process.exit`'s comptime branch (process.sx now imports trace.sx).
- Frame source locations await IR-offset resolution (the comptime analog of
  DWARF), so only function names print today.

examples/252-interp-frames.sx (top-level `#run` drives the dump; exit 0).
Phase E4 (entry-point + stdlib error story) is now 100% complete.

src/ir/inst.zig

@@ -91,6 +91,11 @@ pub const Op = union(enum) {
     /// `false`. Lets stdlib (`process.exit`, `assert`) take a comptime-only
     /// diagnostic branch that dead-codes out of compiled binaries.
     is_comptime,
+    /// ERR E4.1 — `trace.print_interpreter_frames()`. At comptime the interp
+    /// walks its sx call-frame chain and appends it to the output; in compiled
+    /// 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,
     /// 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,