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/pat_extract_partial_drop.ail

@@ -0,0 +1,65 @@
+; Iter 18d.3 — move-aware pattern bindings: let-close emits an
+; inlined per-field dec sequence that skips moved slots and dec's
+; non-moved (wildcarded) pointer-typed slots.
+;
+; Fixture shape:
+;   - Pair carries two `IntList` fields (both pointer-typed).
+;   - main builds a Pair, pattern-matches it binding only the FIRST
+;     field, wildcarding the SECOND.
+;   - The matched binding `a` is consumed by `sum_list`. The
+;     wildcarded slot is logically retained by `p` (no binder owns
+;     it), so at p's let-close it must be dec'd.
+;
+; Expected IR shape under --alloc=rc at p's let-close:
+;   - NO `call void @drop_pat_extract_partial_drop_Pair(ptr <p>)`
+;     (the uniform drop fn is replaced by inline partial drop)
+;   - YES a `getelementptr ... ptr <p>, i64 16` + `load ptr` +
+;     `call void @drop_pat_extract_partial_drop_IntList(ptr <slot1_v>)`
+;     (slot 1 was NOT moved → emit per-field dec)
+;   - YES `call void @ailang_rc_dec(ptr <p>)` (outer cell)
+;   - NO load+dec for slot 0 (moved → skipped)
+;
+; Expected stdout: 6 (1 + 2 + 3 = 6, sum of the first list).
+
+(module pat_extract_partial_drop
+
+  (data IntList
+    (doc "Recursive Int list — boxed.")
+    (ctor Nil)
+    (ctor Cons (con Int) (con IntList)))
+
+  (data Pair
+    (doc "Two IntLists side by side. Both fields are pointer-typed; the test exercises move-tracking against EACH slot independently.")
+    (ctor Pair (con IntList) (con IntList)))
+
+  (fn sum_list
+    (doc "Consume xs, sum its elements.")
+    (type
+      (fn-type
+        (params (con IntList))
+        (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 Pair([1,2,3], [4,5,6]); pattern-bind first slot only; sum first list. The wildcarded second slot's IntList lives only inside the Pair box; closing the Pair's let scope must dec it (non-moved) while skipping the moved first slot.")
+    (type (fn-type (params) (ret (con Unit)) (effects IO)))
+    (params)
+    (body
+      (let p
+        (term-ctor Pair Pair
+          (term-ctor IntList Cons 1
+            (term-ctor IntList Cons 2
+              (term-ctor IntList Cons 3
+                (term-ctor IntList Nil))))
+          (term-ctor IntList Cons 4
+            (term-ctor IntList Cons 5
+              (term-ctor IntList Cons 6
+                (term-ctor IntList Nil)))))
+        (match p
+          (case (pat-ctor Pair a _)
+            (do io/print_int (app sum_list a))))))))