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/list_map_poly.ail

@@ -0,0 +1,62 @@
+; Iter 14b design exhibit, finalised in Iter 14c.
+; Hash-equivalent to list_map_poly.ail.json (round-trip-tested).
+
+(module list_map_poly
+
+  (data List (vars a)
+    (doc "Polymorphic singly-linked list.")
+    (ctor Nil)
+    (ctor Cons a (con List a)))
+
+  (fn inc
+    (doc "Add 1 to an Int.")
+    (type (fn-type (params (con Int)) (ret (con Int))))
+    (params x)
+    (body (app + x 1)))
+
+  (fn map
+    (doc "Polymorphic map: apply f to every element, recursing on the tail.")
+    (type
+      (forall (vars a b)
+        (fn-type
+          (params (fn-type (params a) (ret b))
+                  (con List a))
+          (ret (con List b)))))
+    (params f xs)
+    (body
+      (match xs
+        (case (pat-ctor Nil)
+          (term-ctor List Nil))
+        (case (pat-ctor Cons h t)
+          (term-ctor List Cons
+            (app f h)
+            (app map f t))))))
+
+  (fn print_list
+    (doc "Print each Int on its own line.")
+    (type
+      (fn-type
+        (params (con List (con Int)))
+        (ret (con Unit))
+        (effects IO)))
+    (params xs)
+    (body
+      (match xs
+        (case (pat-ctor Nil)
+          (lit-unit))
+        (case (pat-ctor Cons h t)
+          (seq
+            (do io/print_int h)
+            (tail-app print_list t))))))
+
+  (fn main
+    (doc "Map inc over [1,2,3] via polymorphic List, then print: 2,3,4.")
+    (type (fn-type (params) (ret (con Unit)) (effects IO)))
+    (params)
+    (body
+      (app print_list
+        (app map inc
+          (term-ctor List Cons 1
+            (term-ctor List Cons 2
+              (term-ctor List Cons 3
+                (term-ctor List Nil)))))))))