audit-rt: milestone close — Direction-2 5th test + roadmap entry retired + wording sync

Milestone-close audit found three drift items (architect) plus
two carry-on items; bench all-green.

Fixes (rt.tidy, boss-direct):
- 5th test parse_then_print_then_parse_is_idempotent_on_every_ailx_fixture
  enforces Direction 2 of the Roundtrip Invariant directly. The
  DESIGN.md enforcement list grows from four to five tests.
- docs/roadmap.md P1 'Round-trip completeness invariant' entry
  retired with one-line journal mirror per roadmap convention.
- DESIGN.md wording sync: <16-hex> → <16-lowercase-hex> for the
  Float-bits-hex spelling in the new section (matches §Data model).

Milestone closed clean; five workspace-wide tests anchor the
.ail.json ↔ .ailx bijection plus AST-variant coverage.

docs/journals/2026-05-12-audit-rt.md

@@ -0,0 +1,99 @@
+# audit-rt — Milestone close: Roundtrip Invariant
+
+**Date:** 2026-05-12
+**Milestone:** Roundtrip Invariant (rt.1 + rt.2)
+**Status:** Closed, clean (after rt.tidy fixes)
+
+## Architect drift review
+
+`ailang-architect` reported `drift_found` with four items over
+`daf9f4f..HEAD`:
+
+- `[high]` DESIGN.md §"Roundtrip Invariant" Direction 2 (text →
+  JSON → text idempotency) is stated as a property but not
+  directly enforced by any of the four listed tests. Test 1 covers
+  Direction 1; test 2 covers `.ailx` → JSON ≡ counterpart; test 4
+  covers CLI render → parse. None tests `parse → print → parse`
+  idempotency on a hand-authored `.ailx` text directly. Architect
+  recommendation: claim soften OR add a fifth test.
+- `[high]` `docs/roadmap.md` P1 entry "Round-trip completeness
+  invariant" still `- [ ]` (open) after the milestone closed.
+  Stale per roadmap convention (finished entries → `[x]` then
+  removal with one-line journal mirror).
+- `[low]` DESIGN.md wording: `<16-hex>` in new section vs
+  `<16-lowercase-hex>` in §"Data model". Casing constraint is
+  load-bearing for canonical-byte equality; should be stated
+  identically.
+- `[low]` rt.1 plan errata note (planner emitted ast.rs-mismatching
+  destructure scaffold). Single occurrence absorbed cleanly by the
+  Step-3.4 escape hatch. Not a deeper drift pattern; informational.
+
+## Bench-regression check
+
+`bench/check.py && bench/compile_check.py && bench/cross_lang.py`:
+
+- `check.py`: exit 0; 63 metrics; 0 regressed, 0 improved beyond
+  tolerance, 63 stable.
+- `compile_check.py`: exit 0; 24 metrics; 0 regressed, 0 improved
+  beyond tolerance, 24 stable.
+- `cross_lang.py`: exit 0; 25 metrics; 0 regressed, 0 improved
+  beyond tolerance, 25 stable.
+
+All three bench gates green. Bench was the most plausible source
+of a regression given rt.1 added three new tests but the tests
+are pure readers and are not exercised by the bench harness; the
+green signal matches that expectation.
+
+## Resolution (rt.tidy, boss-direct edits)
+
+Three fixes, mechanical, no plan/implement dispatch:
+
+1. **Direction-2 enforcement** —
+   `crates/ailang-surface/tests/round_trip.rs` gains a third test
+   `parse_then_print_then_parse_is_idempotent_on_every_ailx_fixture`.
+   For every `.ailx` fixture asserts
+   `canonical_bytes(parse(t)) == canonical_bytes(parse(print(parse(t))))`.
+   For the 57 fixtures with a JSON counterpart this is logically
+   derivable from tests 1 + 2; the dedicated test stays robust for
+   future `.ailx` fixtures without a counterpart. DESIGN.md
+   §"Roundtrip Invariant" "Enforcement" list updated from four to
+   five tests; module doc-comment updated from "two complementary
+   checks" to "three complementary checks". Test PASS first-shot
+   across all 57 fixtures.
+
+2. **Roadmap P1 entry removed.** The "Round-trip completeness
+   invariant" milestone entry deleted; this journal entry is the
+   one-line mirror per roadmap convention. The P1 todo "Brainstorm
+   Step 7.5" remains; the P1 Show + print-rewire milestone remains.
+
+3. **Wording fix.** DESIGN.md §"Roundtrip Invariant" Float-bits-hex
+   description changed from `<16-hex>` to `<16-lowercase-hex>` to
+   match §"Data model".
+
+The `[low]` planner-errata carry-on item is retained in the rt.1
+journal "Concerns" section; it will surface again only if a future
+tests-against-AST iteration repeats the pattern.
+
+## Spec acceptance criteria — final check
+
+1. ✓ Top-level `## Roundtrip Invariant` section in DESIGN.md;
+   Decision 6 Constraint 2 carries upward cross-reference. (rt.2)
+2. ✓ `every_ailx_fixture_matches_its_json_counterpart` dynamic in
+   `crates/ailang-surface/tests/round_trip.rs`. (rt.1)
+3. ✓ `crates/ailang-core/tests/schema_coverage.rs` with
+   exhaustive-match visitor; 34/34 variants observed. (rt.1)
+4. ✓ `crates/ail/tests/roundtrip_cli.rs` with BLAKE3 identity
+   over `ail render` → tempfile → `ail parse`. (rt.1)
+5. ✓ `cargo test --workspace` green with all four (now five) tests.
+   (rt.1 + rt.2 + rt.tidy)
+6. ✓ No roundtrip gaps surfaced; no render/parse code changes
+   needed in this milestone. (rt.1)
+7. ✓ Tests are pure readers; the CLI test's only filesystem side
+   effect is `tempfile::TempDir`. (rt.1)
+
+## Outcome
+
+Milestone closed clean. Five workspace-wide tests now anchor the
+`.ail.json` ↔ `.ailx` bijection plus AST-variant coverage; the
+property has a dedicated top-level section in DESIGN.md. No
+follow-up iterations queued in this milestone.

docs/journals/INDEX.md

@@ -18,3 +18,4 @@
 - 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
 - 2026-05-12 — iter rt.2: DESIGN.md anchor for Roundtrip Invariant (top-level section + Decision 6 Constraint 2 upward cross-reference) → 2026-05-12-iter-rt.2.md
+- 2026-05-12 — audit-rt: milestone close (Roundtrip Invariant) — architect drift fixed in rt.tidy (3 items: Direction-2 5th test, roadmap P1 removed, wording sync); bench all-green → 2026-05-12-audit-rt.md