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/gc_stress.ail

@@ -0,0 +1,47 @@
+; Iter 14f stress fixture for the Boehm conservative GC integration.
+; Builds a List Int of length 50 by recursive Cons construction,
+; sums it, prints the sum (1275 = 50*51/2).
+;
+; Bool branching uses the canonical `if` form (Decision 7, restored
+; in Iter 14g): `(if (== n 0) Nil (Cons n (build (- n 1))))`.
+
+(module gc_stress
+
+  (data List (vars a)
+    (doc "Polymorphic singly-linked list (re-declared locally).")
+    (ctor Nil)
+    (ctor Cons a (con List a)))
+
+  (fn build
+    (doc "Build [n, n-1, ..., 1] :: List Int via Cons recursion.")
+    (type
+      (fn-type
+        (params (con Int))
+        (ret (con List (con Int)))))
+    (params n)
+    (body
+      (if (app == n 0)
+        (term-ctor List Nil)
+        (term-ctor List Cons
+          n
+          (app build (app - n 1))))))
+
+  (fn sum_list
+    (doc "Recursively sum a List Int.")
+    (type
+      (fn-type
+        (params (con List (con Int)))
+        (ret (con Int))))
+    (params xs)
+    (body
+      (match xs
+        (case (pat-ctor Nil) 0)
+        (case (pat-ctor Cons h t)
+          (app + h (app sum_list t))))))
+
+  (fn main
+    (doc "Build [50..1], sum, print 1275.")
+    (type (fn-type (params) (ret (con Unit)) (effects IO)))
+    (params)
+    (body
+      (do io/print_int (app sum_list (app build 50))))))