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/lit_pat.ail

@@ -0,0 +1,54 @@
+; Iter 16c — lit patterns at top level and inside Ctor sub-patterns.
+; `classify` matches an Int against three lit arms (0, 1, default).
+; `categorize_first` matches a List<Int>: empty -> -1, head==0 -> 0,
+; otherwise the head value. The latter exercises Pat-Lit nested
+; inside Pat-Ctor's first field. Both shapes are now handled by the
+; 16c desugar pass; codegen sees only If/Match-on-Ctor combinations.
+;
+; Iter 16d update — the chain machinery's terminator is now the
+; polymorphic `__unreachable__` builtin (`forall a. a`, lowered to
+; LLVM `unreachable`) instead of a `Unit` literal. That removed the
+; pre-16d need for a trailing `(case _ 0)` arm in `categorize_first`
+; whose only purpose was to dominate the synthetic `Unit`-typed
+; terminator. The two ctor arms (`Nil` and `Cons h _`) are
+; exhaustive on `IntList`; the chain default is never reached.
+;
+; Expected stdout (one per line): 100, 200, 999, -1, 0, 7.
+
+(module lit_pat
+
+  (data IntList
+    (ctor Nil)
+    (ctor Cons (con Int) (con IntList)))
+
+  (fn classify
+    (doc "Map 0->100, 1->200, anything else->999.")
+    (type (fn-type (params (con Int)) (ret (con Int))))
+    (params n)
+    (body
+      (match n
+        (case (pat-lit 0) 100)
+        (case (pat-lit 1) 200)
+        (case _ 999))))
+
+  (fn categorize_first
+    (doc "Empty -> -1; first element 0 -> 0; otherwise return the head.")
+    (type (fn-type (params (con IntList)) (ret (con Int))))
+    (params xs)
+    (body
+      (match xs
+        (case (pat-ctor Nil) -1)
+        (case (pat-ctor Cons (pat-lit 0) _) 0)
+        (case (pat-ctor Cons h _) h))))
+
+  (fn main
+    (doc "Drive both fns. Expected (per line): 100, 200, 999, -1, 0, 7.")
+    (type (fn-type (params) (ret (con Unit)) (effects IO)))
+    (params)
+    (body
+      (seq (do io/print_int (app classify 0))
+      (seq (do io/print_int (app classify 1))
+      (seq (do io/print_int (app classify 17))
+      (seq (do io/print_int (app categorize_first (term-ctor IntList Nil)))
+      (seq (do io/print_int (app categorize_first (term-ctor IntList Cons 0 (term-ctor IntList Nil))))
+           (do io/print_int (app categorize_first (term-ctor IntList Cons 7 (term-ctor IntList Nil))))))))))))