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.

tests/expected/243-trace-format.txt

@@ -1,5 +1,5 @@
 [stdout] caught BadInput
 error return trace (most recent call last):
-  frame 0: <location pending DWARF> (raw 1)
-  frame 1: <location pending DWARF> (raw 1)
+  leaf at 243-trace-format.sx:20:16
+  mid at 243-trace-format.sx:25:5
 [stdout] recovered; trace buffer now empty (len 0)

tests/expected/244-failable-main.txt

@@ -1,5 +1,5 @@
 v = 10
 error: unhandled error reached main: error.Empty
 error return trace (most recent call last):
-  frame 0: <location pending DWARF> (raw 1)
-  frame 1: <location pending DWARF> (raw 1)
+  inner at 244-failable-main.sx:18:17
+  main at 244-failable-main.sx:26:10

tests/expected/247-failable-or-chain-propagate.txt

@@ -1,8 +1,8 @@
 error: unhandled error reached main: error.A
 error return trace (most recent call last):
-  frame 0: <location pending DWARF> (raw 1)
-  frame 1: <location pending DWARF> (raw 1)
-  frame 2: <location pending DWARF> (raw 1)
-  frame 3: <location pending DWARF> (raw 1)
-  frame 4: <location pending DWARF> (raw 1)
-  frame 5: <location pending DWARF> (raw 1)
+  fa at 247-failable-or-chain-propagate.sx:14:17
+  main at 247-failable-or-chain-propagate.sx:19:10
+  fa at 247-failable-or-chain-propagate.sx:14:17
+  main at 247-failable-or-chain-propagate.sx:19:10
+  fa at 247-failable-or-chain-propagate.sx:14:17
+  main at 247-failable-or-chain-propagate.sx:19:10