Connect the E3.1 buffer to codegen. Push sites: `raise` (always escapes — push
before cleanup) and `try`'s propagation branch (the failure that escapes to the
caller). Clear sites: `catch` handler entry (via runCatchBody, error path only),
the `or value` terminator's failure branch, and a destructure that binds a
failable's error slot — so an absorbed failure leaves no residue.
Helpers in lower.zig: emitTracePush / emitTraceClear (call getTraceFids, no-op
when traces are off), tracesEnabled (opt_level == .none/.less — `sx run`
defaults to -O0, so on in dev; .default/.aggressive are release → off, zero
overhead), and placeholderTraceFrame (a nonzero u64 until DWARF/E3.0 supplies
real PCs and E3.3 resolves them).
Verified end-to-end via a #foreign sx_trace_len probe: catch/or/multi-slot-
destructure drive len back to 0; release (--opt default) emits no push/clear at
all (debug showed a residual where release showed 0).
examples/241-error-trace-buffer.sx is a focused regression (white-box: reads
sx_trace_len directly, pending E3.3's public trace.print_current).
KNOWN GAP (documented, deferred to the E1.8 flow-check binding-site work): a
single-binding capture of a PURE failable (`er := pure_failable()`, not a
comma destructure) goes through lowerVarDecl, not lowerDestructureDecl, so it
doesn't clear — the trace over-retains until the next absorbing site. Harmless
today (nothing reads the buffer at function exit yet) but wrong per spec.
Gates: zig build, zig build test, bash tests/run_examples.sh (278 passed; lone
failure is the user's uncommitted 213-canonical-map pack WIP).
