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.

bench/orchestrator-stats/2026-05-12-iter-rt.1.json

@@ -0,0 +1,12 @@
+{
+  "iter_id": "rt.1",
+  "date": "2026-05-12",
+  "mode": "standard",
+  "outcome": "DONE",
+  "tasks_total": 4,
+  "tasks_completed": 4,
+  "reloops_per_task": { "1": 0, "2": 0, "3": 0, "4": 0 },
+  "review_loops_spec": 0,
+  "review_loops_quality": 0,
+  "blocked_reason": null
+}