iter form-a.1 (Tasks 6-12): milestone close

Second half of the form-a-default-authoring milestone-close iter
(Boss-decided strategy C, big-bang). All seven tasks DONE; cargo
test --workspace green at every per-task boundary.

T6 — Bench-driver suffix flip from .ail.json to .ail across 4
Python scripts + run.sh. compile_check.py + cross_lang.py exit 0.

T7 — Re-author e2e.rs raw-JSON-inspect tests:
- diff_detects_changed_def — derive sum.ail.json on-the-fly via
  `ail parse examples/sum.ail` into tempdir, then mutate + diff.
- borrow_own_demo_modes_are_metadata_only — same pattern.
- reuse_as_demo_under_rc_uses_inplace_rewrite — same pattern.
- render_parse_round_trip_canonical — RETIRED (subsumed by T1's
  cli_parse_then_render_then_parse_is_idempotent over whole corpus).
- ail_run_accepts_ail_source_with_same_stdout_as_ail_json —
  re-authored to derive hello.ail.json in a per-process tempdir
  from hello.ail via `ail parse`, then assert dual-form stdout.

T8 — Bulk-delete 156 non-carve-out .ail.json. Inventory:
8 .ail.json (carve-outs, alphabetical: broken_unbound + prelude
+ 3× test_22b2_* + 3× test_ct1_*) + 157 .ail. carve_out_inventory
test un-#[ignore]'d and green. Forward-pulled 20 repairs that the
T1-5 dispatch's recon missed (12 Group-B suffix + 5 Group-A
load_workspace + 3 ail_run sites). Also forward-pulled T9 Step 5
(schema_coverage corpus flip from .ail.json to .ail) to satisfy
T8's green-gate.

T9 — Retire obsolete roundtrip tests:
- print_then_parse_round_trips_every_fixture (round_trip.rs)
- every_ail_fixture_matches_its_json_counterpart (round_trip.rs)
- cli_render_then_parse_preserves_canonical_bytes_on_every_fixture
- Dead helpers: list_json_fixtures ×2, round_trip_one,
  strip_trailing_newlines.
Schema-coverage corpus already flipped in T8 (forward-pull).

T10 — DESIGN.md §"Roundtrip Invariant" (lines 2027-2109) restated
with parse-determinism + idempotency + CLI-pipeline-idempotency +
carve-out-anchor framing. Five surviving enforcement tests named.
§"Float literals" and §"Why anchored at top level" preserved.

T11 — §A4 doctrine edits: CLAUDE.md:5-6 + DESIGN.md:465-466.
Canonical form remains JSON-AST; authoring projection is .ail;
build derives JSON-AST in-process via ailang_surface::parse.

T12 — Milestone close:
- WhatsNew entry: user-facing language, lead with the change.
- Roadmap: [milestone] form-a struck [x] with closing note.
- Final inventory verified: 8 .ail.json + 157 .ail.
- Final cargo test --workspace: 557 passed, 0 failed, 3 ignored.
- bench/compile_check.py + bench/cross_lang.py: exit 0.

Test math: pre-iter 558 baseline + 3 new T1 tests = 561, − 1
(T7 retire) − 3 (T9 retire) = 557 final.

INDEX.md appended with the full iter summary covering T1-T12 (the
T1-5 commit at 77b28ad deferred the INDEX line to full-iter close).

Milestone [Form-A as the default authoring surface] structurally
closed. The compile-time-embed carve-out (prelude.ail.json) is
the subject of the queued follow-up milestone [Prelude embed:
Form-A as compile-time source]. audit-form-a runs as the next
dispatch.

bench/run.sh

@@ -65,7 +65,7 @@ fixtures=(bench_list_sum bench_tree_walk bench_closure_chain bench_hof_pipeline
 modes=(gc bump rc)
 echo ">>> compiling fixtures (-O2)"
 for f in "${fixtures[@]}"; do
-    src="$ROOT/examples/$f.ail.json"
+    src="$ROOT/examples/$f.ail"
     [[ -f "$src" ]] || { echo "missing fixture: $src" >&2; exit 1; }
     for m in "${modes[@]}"; do
         bin="$OUTDIR/${f}_${m}"
@@ -179,8 +179,8 @@ done
 # was added in 18f.2 and may not exist on older branches that share
 # this script).
 LAT_HARNESS="$ROOT/bench/latency_harness.py"
-LAT_IMPL_SRC="$ROOT/examples/bench_latency_implicit.ail.json"
-LAT_EXPL_SRC="$ROOT/examples/bench_latency_explicit.ail.json"
+LAT_IMPL_SRC="$ROOT/examples/bench_latency_implicit.ail"
+LAT_EXPL_SRC="$ROOT/examples/bench_latency_explicit.ail"
 if [[ -x "$LAT_HARNESS" && -f "$LAT_IMPL_SRC" && -f "$LAT_EXPL_SRC" ]]; then
     echo
     echo ">>> latency bench (PTY inter-arrival, 1000 samples per arm)"