iter rt.1: roundtrip-invariant audit tests — 3 new tests, all PASS first-shot

Three new reader-only tests pin the .ail.json / .ailx bijection
plus AST-variant coverage. All three passed first observation
across the current corpus — no roundtrip, schema-coverage, or
CLI-drift gaps surfaced.

- every_ailx_fixture_matches_its_json_counterpart (replaces the
  3-pair handwritten exhibits): 57 paired .ailx fixtures pass.
- every_ast_variant_is_observed_in_the_fixture_corpus (new): 34/34
  AST variants exercised across 136 fixtures; exhaustive matches
  without _ wildcard so AST drift fails the build.
- cli_render_then_parse_preserves_canonical_bytes_on_every_fixture
  (new): 136 fixtures round-trip through ail render → tempfile →
  ail parse with BLAKE3 identity on canonical bytes.

No production code, no DESIGN.md changes (those follow in later
iterations). Tests are pure readers of the repo per spec
acceptance #7 — tempfile crate added as workspace dep for the CLI
roundtrip's intermediate file outside the repo.

docs/journals/INDEX.md

@@ -16,3 +16,4 @@
 - 2026-05-11 — iter 23.4: mono-pass unification (class methods + polymorphic free fns in one fixpoint; codegen-time specialiser removed) → 2026-05-11-iter-23.4.md
 - 2026-05-12 — iter 23.5: prelude free fns (ne/lt/le/gt/ge) + E2E (positive / user-ADT / Float-NoInstance); milestone 23 closed → 2026-05-12-iter-23.5.md
 - 2026-05-12 — audit-23: milestone 23 close (architect clean, compile_check ratified per H2 / 5-fn workload), DESIGN.md stub-fix tidy → 2026-05-12-audit-23.md
+- 2026-05-12 — iter rt.1: roundtrip invariant audit tests (3 new tests + ailx-pair dynamic); all PASS first-shot, no gaps surfaced → 2026-05-12-iter-rt.1.md