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/DESIGN.md

@@ -1781,8 +1781,8 @@ without changing the identity of the module it produces.
 
 Float literals carry an IEEE-754 bit pattern, not a decimal
 approximation. The canonical encoding is
-`{"kind":"float","bits":"<16-hex>"}` and the surface emits the
-same bits-hex string. NaN, ±Inf, signed zero, and subnormals all
+`{"kind":"float","bits":"<16-lowercase-hex>"}` and the surface
+emits the same bits-hex string. NaN, ±Inf, signed zero, and subnormals all
 round-trip exactly because the JSON-number path is bypassed (see
 §"Float semantics", "Form-A serialisation"). Floats are not an
 exception to the invariant — the bits-hex encoding is the
@@ -1790,7 +1790,7 @@ mechanism that keeps them *inside* it.
 
 ### Enforcement
 
-The invariant is workspace-CI-enforced by four tests, each
+The invariant is workspace-CI-enforced by five tests, each
 operating on the `examples/` corpus via dynamic `read_dir`
 collection (no hardcoded fixture list, so newly added fixtures
 inherit the gate automatically):
@@ -1798,6 +1798,9 @@ inherit the gate automatically):
 - `crates/ailang-surface/tests/round_trip.rs::print_then_parse_round_trips_every_fixture`
   — for every `.ail.json`, `print` then `parse` produces canonical-
   byte-equal output. Direction 1 above.
+- `crates/ailang-surface/tests/round_trip.rs::parse_then_print_then_parse_is_idempotent_on_every_ailx_fixture`
+  — for every `.ailx` text `t`, `parse(t)` and `parse(print(parse(t)))`
+  produce canonical-byte-equal AST. Direction 2 above.
 - `crates/ailang-surface/tests/round_trip.rs::every_ailx_fixture_matches_its_json_counterpart`
   — for every `.ailx` with a same-stem `.ail.json` counterpart,
   `parse` of the text yields canonical bytes equal to the JSON