iter ext-rename: .ailx → .ail across the live toolchain

The surface-form file extension changes from .ailx to .ail. AILang's
authoring surface now uses the same .ail stem as its canonical JSON
form (.ail.json), giving the language a single coherent extension
family: .ail is the LLM-authored Form A, .ail.json is the canonical
JSON-AST Form B.

Scope (touched):
- 61 example renames examples/**/*.ailx → .ail (git mv)
- 1 rename experiments/.../rendered/ailx.md → ail.md
- 35 content-edited live-toolchain files (crates/, docs/DESIGN.md,
  docs/roadmap.md, docs/PROSE_ROUNDTRIP.md, skills/, bench/reference/*.c,
  experiment crates under experiments/.../{render,harness,master})
- Experiment-crate cohort rename Cohort::Ailx → Cohort::Ail,
  Form::Ailx → Form::Ail, per_cohort/ailx → per_cohort/ail,
  {form-only: ailx} → {form-only: ail}, ```ailx → ```ail

Out of scope (deliberately untouched, to preserve honest history):
- docs/journal-archive.md (content-frozen per CLAUDE.md)
- docs/journals/, docs/specs/, docs/plans/, bench/orchestrator-stats/
- experiments/.../runs/ (frozen LLM-output artefacts; models actually
  saw .ailx — renaming would falsify the experimental record)

Verification: cargo build/test --workspace green; experiment crate
cargo test green; bench/check.py + compile_check.py + cross_lang.py
all 0-regressed; negative grep for ailx|Ailx|AILX outside the
out-of-scope paths returns zero matches.

Opens immediate follow-up: roadmap.md P2 todo `ail check`/build/run
accept .ail extension — after this rename, .ail is canonical
authoring surface but the CLI still produces a misleading JSON-parse
error on `ail check foo.ail`. That's the next iter.

examples/nested_let_rec.ail

@@ -0,0 +1,53 @@
+; Iter 16b.7 — nested LetRec mutual capture (params, not name).
+;
+; `nested_sum(n)` returns the sum 1 + 2 + ... + n by combining two
+; recursive helpers. The OUTER let-rec `outer(i)` iterates i over
+; 1..=n; for each i it calls the INNER let-rec `inner(j)` to count
+; how many integers are in 1..=i (= i). The total is then
+; 1 + 2 + ... + n = n*(n+1)/2.
+;
+; The shape exercises:
+; - outer captures `n` from `nested_sum`'s fn-params (16b.2 case).
+; - inner captures `i` from outer LetRec's PARAMS (16b.7 case
+;   under iter — outer LetRec's params are KnownType in body
+;   scope, so the inner lift sees a resolvable capture type).
+; - inner does NOT capture outer's NAME (still rejected in 16b.7).
+;
+; Expected stdout (one per line):
+;   nested_sum(1) = 1
+;   nested_sum(3) = 6
+;   nested_sum(5) = 15
+
+(module nested_let_rec
+
+  (fn nested_sum
+    (doc "Sum 1 + 2 + ... + n by nested LetRec helpers; inner captures outer's param i.")
+    (type (fn-type (params (con Int)) (ret (con Int))))
+    (params n)
+    (body
+      (let-rec outer
+        (params i)
+        (type (fn-type (params (con Int)) (ret (con Int))))
+        (body
+          (if (app > i n)
+            0
+            (app +
+              (let-rec inner
+                (params j)
+                (type (fn-type (params (con Int)) (ret (con Int))))
+                (body
+                  (if (app > j i)
+                    0
+                    (app + 1 (app inner (app + j 1)))))
+                (in (app inner 1)))
+              (app outer (app + i 1)))))
+        (in (app outer 1)))))
+
+  (fn main
+    (doc "Drive nested_sum at 1, 3, 5. Expected (per line): 1, 6, 15.")
+    (type (fn-type (params) (ret (con Unit)) (effects IO)))
+    (params)
+    (body
+      (seq (do io/print_int (app nested_sum 1))
+      (seq (do io/print_int (app nested_sum 3))
+           (do io/print_int (app nested_sum 5)))))))