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_maybe.ail

@@ -0,0 +1,64 @@
+; Iter 15a — first stdlib module: optional values.
+; Polymorphic Maybe with four pure combinators.
+
+(module std_maybe
+
+  (data Maybe (vars a)
+    (doc "Polymorphic optional value: either Just<a> or Nothing.")
+    (ctor Nothing)
+    (ctor Just a))
+
+  (fn from_maybe
+    (doc "Project out of Maybe<a> with a default for the Nothing case.")
+    (type
+      (forall (vars a)
+        (fn-type
+          (params a (con Maybe a))
+          (ret a))))
+    (params default m)
+    (body
+      (match m
+        (case (pat-ctor Just x) x)
+        (case (pat-ctor Nothing) default))))
+
+  (fn is_some
+    (doc "Returns true iff m is Just<_>.")
+    (type
+      (forall (vars a)
+        (fn-type
+          (params (con Maybe a))
+          (ret (con Bool)))))
+    (params m)
+    (body
+      (match m
+        (case (pat-ctor Just _) true)
+        (case (pat-ctor Nothing) false))))
+
+  (fn is_none
+    (doc "Returns true iff m is Nothing.")
+    (type
+      (forall (vars a)
+        (fn-type
+          (params (con Maybe a))
+          (ret (con Bool)))))
+    (params m)
+    (body
+      (match m
+        (case (pat-ctor Just _) false)
+        (case (pat-ctor Nothing) true))))
+
+  (fn map_maybe
+    (doc "Apply f to the wrapped value, or pass Nothing through.")
+    (type
+      (forall (vars a b)
+        (fn-type
+          (params (fn-type (params a) (ret b))
+                  (con Maybe a))
+          (ret (con Maybe b)))))
+    (params f m)
+    (body
+      (match m
+        (case (pat-ctor Just x)
+          (term-ctor Maybe Just (app f x)))
+        (case (pat-ctor Nothing)
+          (term-ctor Maybe Nothing))))))