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/std_pair.ail

@@ -0,0 +1,77 @@
+; Iter 15f — fourth stdlib module: product types.
+; Pair<a, b> is the canonical 2-type-var, single-ctor product. No
+; recursion in the data def or any combinator. Combinators here
+; build on the new (Iter 16a) nested-Ctor-pattern support: `swap`
+; flattens via `(pat-ctor MkPair x y)` and the eliminator `pair`
+; matches the same shape directly without an outer var-binding.
+
+(module std_pair
+
+  (data Pair (vars a b)
+    (doc "Polymorphic product: a single ctor MkPair holding one value of each parameter.")
+    (ctor MkPair a b))
+
+  (fn fst
+    (doc "First projection. Returns the `a` payload.")
+    (type
+      (forall (vars a b)
+        (fn-type
+          (params (con Pair a b))
+          (ret a))))
+    (params p)
+    (body
+      (match p
+        (case (pat-ctor MkPair x _) x))))
+
+  (fn snd
+    (doc "Second projection. Returns the `b` payload.")
+    (type
+      (forall (vars a b)
+        (fn-type
+          (params (con Pair a b))
+          (ret b))))
+    (params p)
+    (body
+      (match p
+        (case (pat-ctor MkPair _ y) y))))
+
+  (fn swap
+    (doc "Exchange the components: Pair<a, b> -> Pair<b, a>.")
+    (type
+      (forall (vars a b)
+        (fn-type
+          (params (con Pair a b))
+          (ret (con Pair b a)))))
+    (params p)
+    (body
+      (match p
+        (case (pat-ctor MkPair x y)
+          (term-ctor Pair MkPair y x)))))
+
+  (fn map_first
+    (doc "Apply f to the first component, leave the second intact.")
+    (type
+      (forall (vars a b c)
+        (fn-type
+          (params (fn-type (params a) (ret c))
+                  (con Pair a b))
+          (ret (con Pair c b)))))
+    (params f p)
+    (body
+      (match p
+        (case (pat-ctor MkPair x y)
+          (term-ctor Pair MkPair (app f x) y)))))
+
+  (fn map_second
+    (doc "Apply f to the second component, leave the first intact.")
+    (type
+      (forall (vars a b c)
+        (fn-type
+          (params (fn-type (params b) (ret c))
+                  (con Pair a b))
+          (ret (con Pair a c)))))
+    (params f p)
+    (body
+      (match p
+        (case (pat-ctor MkPair x y)
+          (term-ctor Pair MkPair x (app f y)))))))