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_let_alias_implicit_param.ail

@@ -0,0 +1,79 @@
+; Iter 18g tidy follow-up RED-test fixture: a let-binder whose
+; value is a Term::Var referencing a non-Own fn-param must
+; inherit the param's mode for codegen's drop-emission gates.
+;
+; The carve-out documented in 18d.4's Iter A fix: the gate
+; checks `scrutinee_is_owned` against `current_param_modes`,
+; but only fn-params are recorded there. A let-binder aliasing
+; an Implicit-mode fn-param passes the gate (its lookup
+; misses, default = owned), so the arm-close drop fires on
+; memory the caller still references.
+;
+; Pattern: `pin_aliased` takes `t` as Implicit-mode (default,
+; no annotation); aliases it via `(let a t ...)`; matches `a`.
+; In the TNode arm the pattern-bindings `l` / `r` would be
+; dec'd at arm close because `a` looks owned to the gate —
+; corrupting the caller's tree.
+;
+; Pre-fix under --alloc=rc: refcount underflow at the second
+; iteration of `loop`, where the recursive call into
+; `pin_aliased(t)` re-loads cells the previous iteration
+; freed.
+; Post-fix: clean exit, output `0`. Stats are not the focus
+; here (the caller's tree leaks under the Implicit-mode
+; back-compat lane); the test asserts only correctness.
+
+(module rc_let_alias_implicit_param
+
+  (data Tree
+    (ctor TLeaf)
+    (ctor TNode (con Int) (con Tree) (con Tree)))
+
+  (fn build
+    (type
+      (fn-type
+        (params (con Int))
+        (ret (own (con Tree)))))
+    (params d)
+    (body
+      (if (app == d 0)
+        (term-ctor Tree TLeaf)
+        (term-ctor Tree TNode 1
+          (app build (app - d 1))
+          (app build (app - d 1))))))
+
+  ; Pin via let-alias: the alias `a` is a Var-binding on the
+  ; Implicit-mode (default) param `t`. Codegen's gate must
+  ; recognise that `a` inherits `t`'s mode and skip the arm-
+  ; close drop that would otherwise fragment the caller's
+  ; tree.
+  (fn pin_aliased
+    (type
+      (fn-type
+        (params (con Tree))
+        (ret (con Int))))
+    (params t)
+    (body
+      (let a t
+        (match a
+          (case (pat-ctor TLeaf) 0)
+          (case (pat-ctor TNode v l r) 1)))))
+
+  (fn loop
+    (type
+      (fn-type
+        (params (con Int) (con Tree))
+        (ret (con Int))))
+    (params n t)
+    (body
+      (if (app == n 0)
+        0
+        (let _v (app pin_aliased t)
+          (app loop (app - n 1) t)))))
+
+  (fn main
+    (type (fn-type (params) (ret (con Unit)) (effects IO)))
+    (params)
+    (body
+      (let t (app build 2)
+        (do io/print_int (app loop 3 t))))))