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/local_rec_match_capture.ail

@@ -0,0 +1,59 @@
+; Iter 16b.4 — LetRec captures of match-arm pattern bindings.
+; `count_below(p)` takes a `Pair Int Int` (threshold, n), pattern-
+; matches on `(MkPair threshold n)`, and inside that match arm runs
+; a recursive helper `loop` that captures BOTH match-arm bindings
+; (`threshold` and `n`) and counts how many integers in 1..=n are
+; strictly less than `threshold`.
+;
+; The 16b.2/16b.3 desugar passes cannot lift this LetRec — both
+; `threshold` and `n` are bound by a `Pattern::Ctor` Var sub-
+; pattern (`ScopeEntry::MatchArm`). Before 16b.4 this panicked at
+; desugar time. Now the desugar pass leaves the LetRec in place;
+; the post-typecheck `lift_letrecs` pass walks the enclosing match,
+; resolves each pattern binding's type via constructor-field
+; substitution against the matched ctor's declared field types
+; (here: `Pair Int Int`'s ctor `MkPair` has fields `[Int, Int]`,
+; and the scrutinee's `Type::Con.args` are also `[Int, Int]`, so
+; substitution is a no-op and both bindings get type `Int`), and
+; lifts to `loop$lr_0(i: Int, threshold: Int, n: Int) -> Int`.
+;
+; The match has a single ctor and no catch-all; the chain machinery
+; treats the only arm as default-dominating since there's no `_`
+; arm needed for an exhaustive single-ctor ADT (Pair has one ctor).
+;
+; Expected stdout (one per line):
+;   count_below(MkPair 10 0)  = 0
+;   count_below(MkPair 10 5)  = 5  (1..=5 all below 10)
+;   count_below(MkPair 10 15) = 9  (1..=9 below 10, 10..=15 not)
+
+(module local_rec_match_capture
+
+  (data Pair (vars a b)
+    (ctor MkPair a b))
+
+  (fn count_below
+    (doc "Match-arm captures `threshold` and `n`; inner LetRec captures both.")
+    (type (fn-type (params (con Pair (con Int) (con Int))) (ret (con Int))))
+    (params p)
+    (body
+      (match p
+        (case (pat-ctor MkPair threshold n)
+          (let-rec loop
+            (params i)
+            (type (fn-type (params (con Int)) (ret (con Int))))
+            (body
+              (if (app > i n)
+                0
+                (if (app < i threshold)
+                  (app + 1 (app loop (app + i 1)))
+                  (app loop (app + i 1)))))
+            (in (app loop 1)))))))
+
+  (fn main
+    (doc "Drive count_below at MkPair 10 {0,5,15}. Expected: 0, 5, 9.")
+    (type (fn-type (params) (ret (con Unit)) (effects IO)))
+    (params)
+    (body
+      (seq (do io/print_int (app count_below (term-ctor Pair MkPair 10 0)))
+      (seq (do io/print_int (app count_below (term-ctor Pair MkPair 10 5)))
+           (do io/print_int (app count_below (term-ctor Pair MkPair 10 15))))))))