ERR/E3.3: trace formatting (library/modules/trace.sx) + catch-clear timing fix

The trace formatter, unblocked now that 0057 is fixed.

- library/modules/trace.sx: to_string() walks the trace buffer (sx_trace_len /
  frame_at / truncated) and renders "error return trace ..." with one line per
  frame; print_current() writes it to stderr (libc write(2, ...)). Frame
  locations are "<location pending DWARF>" until E3.0 resolves PCs; count +
  ordering + the overflow note are already meaningful.

- Catch-clear timing fix (lowerCatch): move the absorption clear from
  runCatchBody ENTRY to the handler's non-diverging EXIT (both the pure and
  value-carrying paths). This reconciles the two PLAN-ERR statements that
  conflicted — §clear-points "buffer cleared before the catch body" vs
  §catch-over-or "frames still in the buffer when the body runs". Exit-clear
  satisfies both: the handler can inspect the trace (trace.print_current()
  shows the chain), and the buffer is empty once the handler completes. A
  diverging body (raise/return) keeps/discards on its own path.

- examples/243-trace-format.sx: catch handler prints the tag + the 2-frame
  trace, then shows the buffer is empty after. examples/241 updated: the
  handler now observes len=2 (was 0 under the buggy entry-clear).

Gates: zig build, zig build test, bash tests/run_examples.sh (280 passed; lone
failure is the user's uncommitted 213-canonical-map pack WIP).

tests/expected/241-error-trace-buffer.txt

@@ -1,3 +1,3 @@
-in catch: len=0
+in catch: len=2
 after catch: len=0
 after success: len=0

tests/expected/243-trace-format.exit

@@ -0,0 +1 @@
+0

tests/expected/243-trace-format.txt

@@ -0,0 +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)
+[stdout] recovered; trace buffer now empty (len 0)