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/rc_match_arm_partial_drop_leak.ail

@@ -0,0 +1,22 @@
+(module rc_match_arm_partial_drop_leak
+  (data Wrap
+    (ctor MkWrap (con Int)))
+  (data Cell
+    (ctor MkCell (con Wrap) (con Wrap)))
+  (data Pair
+    (ctor MkPair (con Cell) (con Cell)))
+  (fn build_pair
+    (type (fn-type (params (con Int)) (ret (own (con Pair)))))
+    (params n)
+    (body (term-ctor Pair MkPair (term-ctor Cell MkCell (term-ctor Wrap MkWrap n) (term-ctor Wrap MkWrap 2)) (term-ctor Cell MkCell (term-ctor Wrap MkWrap 3) (term-ctor Wrap MkWrap 4)))))
+  (fn use_first
+    (suppress (code "over-strict-mode") (because "RC codegen test: exercises Iter A outer-arm-close partial-drop"))
+    (type (fn-type (params (own (con Pair))) (ret (con Int))))
+    (params p)
+    (body (match p
+      (case (pat-ctor MkPair a b) (match a
+        (case (pat-ctor MkCell w1 _) 1))))))
+  (fn main
+    (type (fn-type (params) (ret (con Unit)) (effects IO)))
+    (params)
+    (body (do io/print_int (app use_first (app build_pair 1))))))