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.

crates/ailang-core/tests/carve_out_inventory.rs

@@ -31,7 +31,6 @@ fn examples_dir() -> std::path::PathBuf {
 }
 
 #[test]
-#[ignore = "GREEN-ratified by Task 8 (.ail.json deletion); RED until then"]
 fn examples_ail_json_inventory_matches_carve_outs() {
     let expected: BTreeSet<&str> = EXPECTED.iter().copied().collect();
     let mut actual: BTreeSet<String> = BTreeSet::new();

crates/ailang-core/tests/schema_coverage.rs

@@ -326,7 +326,7 @@ fn examples_dir() -> PathBuf {
     crate_dir.parent().unwrap().parent().unwrap().join("examples")
 }
 
-fn list_json_fixtures() -> Vec<PathBuf> {
+fn list_ail_fixtures() -> Vec<PathBuf> {
     let dir = examples_dir();
     let mut paths: Vec<PathBuf> = std::fs::read_dir(&dir)
         .unwrap_or_else(|e| panic!("read_dir({}): {e}", dir.display()))
@@ -335,7 +335,7 @@ fn list_json_fixtures() -> Vec<PathBuf> {
         .filter(|p| {
             p.file_name()
                 .and_then(|n| n.to_str())
-                .map(|n| n.ends_with(".ail.json"))
+                .map(|n| n.ends_with(".ail"))
                 .unwrap_or(false)
         })
         .collect();
@@ -345,10 +345,10 @@ fn list_json_fixtures() -> Vec<PathBuf> {
 
 #[test]
 fn every_ast_variant_is_observed_in_the_fixture_corpus() {
-    let fixtures = list_json_fixtures();
+    let fixtures = list_ail_fixtures();
     assert!(
         !fixtures.is_empty(),
-        "no .ail.json fixtures found under {}",
+        "no .ail fixtures found under {}",
         examples_dir().display()
     );
 
@@ -356,7 +356,7 @@ fn every_ast_variant_is_observed_in_the_fixture_corpus() {
     let mut load_failures = Vec::<String>::new();
 
     for path in &fixtures {
-        match ailang_core::load_module(path) {
+        match ailang_surface::load_module(path) {
             Ok(m) => visit_module(&m, &mut observed),
             Err(e) => load_failures.push(format!("{}: {e}", path.display())),
         }