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/ct_1_ordering_signum.ail

@@ -0,0 +1,55 @@
+; Fieldtest — canonical-type-names, axis 4: canonical happy path.
+;
+; Reduce a list of Int to a 'sign profile': for each element, print
+; -1, 0, or +1 according to its compare-to-zero result. Exercises:
+;   - cross-module reference to prelude.Ordering at use site
+;   - pattern match against prelude.Ordering's three constructors LT/EQ/GT
+;   - returning a freshly chosen Int from each arm
+;   - simple recursion over a local List ADT
+;
+; Expected stdout (one per line):
+;   -1   ; signum -3
+;    0   ; signum  0
+;    1   ; signum  7
+;   -1   ; signum -1
+;    1   ; signum  2
+
+(module ct_1_ordering_signum
+
+  (data IntList
+    (doc "Local Int-list ADT; nothing fancy.")
+    (ctor INil)
+    (ctor ICons (con Int) (con IntList)))
+
+  (fn signum
+    (doc "Map an Int to -1/0/+1 via prelude.compare. Returns the sign as an Int.")
+    (type (fn-type (params (con Int)) (ret (con Int))))
+    (params n)
+    (body
+      (match (app compare n 0)
+        (case (pat-ctor LT) (app - 0 1))
+        (case (pat-ctor EQ) 0)
+        (case (pat-ctor GT) 1))))
+
+  (fn drain
+    (doc "Print signum of every element of xs, in list order.")
+    (type (fn-type (params (con IntList)) (ret (con Unit)) (effects IO)))
+    (params xs)
+    (body
+      (match xs
+        (case (pat-ctor INil) (lit-unit))
+        (case (pat-ctor ICons h t)
+          (seq (do io/print_int (app signum h))
+               (tail-app drain t))))))
+
+  (fn main
+    (type (fn-type (params) (ret (con Unit)) (effects IO)))
+    (params)
+    (body
+      (app drain
+        (term-ctor IntList ICons (app - 0 3)
+        (term-ctor IntList ICons 0
+        (term-ctor IntList ICons 7
+        (term-ctor IntList ICons (app - 0 1)
+        (term-ctor IntList ICons 2
+        (term-ctor IntList INil))))))))))