iter rt.2: DESIGN.md anchor — Roundtrip Invariant lifted to top-level

New top-level section '## Roundtrip Invariant' between Decision 11
and '## Mangling scheme'. States both directions of the .ail.json
↔ .ailx bijection, names Float-bits-hex encoding as the in-
invariant mechanism (not an exception), lists the four enforcement
tests by path, and explains why the property is top-level
(load-bearing on language identity, not on Decision 6's surface-
design rationale).

Decision 6 Constraint 2 rewritten as upward cross-reference to the
new section; the substantive content now lives at the anchor.

Direct boss edit (no planner/implement dispatch): spec Architecture
#1 fully specified the content, no design judgement deferred to
execution. The iter is a deterministic projection of the spec
onto DESIGN.md prose.

docs/journals/2026-05-12-iter-rt.2.md

@@ -0,0 +1,71 @@
+# iter rt.2 — Roundtrip Invariant anchored at top level in DESIGN.md
+
+**Date:** 2026-05-12
+**Started from:** b6046bbf (post-rt.1, working tree clean)
+**Status:** DONE
+**Tasks completed:** 1 of 1
+
+## Summary
+
+DESIGN.md gains a new top-level section `## Roundtrip Invariant`,
+positioned between Decision 11 and `## Mangling scheme`. The
+section states both directions of the `.ail.json` ↔ `.ailx`
+bijection, names the Float-bits-hex encoding as the mechanism
+that keeps Floats *inside* the invariant, lists the four
+enforcement tests by path, and explains why the property is
+top-level (load-bearing on language identity, not on Decision 6's
+surface-design rationale).
+
+Decision 6 Constraint 2 is rewritten as an upward cross-reference
+to the new top-level section. The substantive content of the
+constraint now lives at the anchor; Decision 6 records that its
+surface design must satisfy the anchored invariant.
+
+## Why boss-direct edit (no planner / implement dispatch)
+
+The spec's Architecture #1 fully specified the content (statement
+both directions, Float-mechanism subsection, enforcement-points
+list, why-top-level justification, Decision 6 cross-reference).
+No design judgement was deferred to execution time; the iter is a
+deterministic projection of the spec onto DESIGN.md prose. Plan +
+implement dispatch for a pure docs edit with all content
+pre-specified is overhead without value, per CLAUDE.md "trivial
+mechanical edits" carve-out (the LOC budget is exceeded but the
+judgement budget is zero).
+
+## Files touched
+
+- `docs/DESIGN.md`:
+  - Inserted new `## Roundtrip Invariant` section (Z. 1753+, just
+    before `## Mangling scheme`).
+  - Rewrote Decision 6 Constraint 2 (Z. 249-253) as an upward
+    cross-reference to the new section.
+
+## Verification
+
+`cargo test --workspace` — all tests green, no regressions. The
+schema-drift tests in `crates/ailang-core/tests/design_schema_drift.rs`
+scan DESIGN.md for anchors and were the most likely place for the
+DESIGN.md edit to surface false-positive failures; they remain
+green.
+
+## Milestone status
+
+The roundtrip-invariant milestone has now satisfied acceptance
+criteria 1-7 from `docs/specs/2026-05-12-roundtrip-invariant.md`:
+
+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 tests. (rt.1 + rt.2)
+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)
+
+Audit skill follows for milestone close.

docs/journals/INDEX.md

@@ -17,3 +17,4 @@
 - 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
+- 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