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.

crates/ailang-core/specs/form_a.md

@@ -2,7 +2,7 @@
 
 Form-A is the canonical textual surface of AILang. It is the form that
 LLMs generate when asked to produce or edit AILang code, and the form
-that `ail parse <file>.ailx` reads. The inverse direction — printing
+that `ail parse <file>.ail` reads. The inverse direction — printing
 JSON-AST as Form-A — is `ail render <file>.ail.json`. Round-trip
 through this pair is the gating contract: `parse(render(m)) == m`
 for every well-formed module.
@@ -26,7 +26,7 @@ Lisp-style S-expression dress that:
 - omits structural noise (no field tags; positions carry meaning)
 - has a real parser with positional error messages
 - round-trips through `ail render` ↔ `ail parse` losslessly
-- is the form every existing `examples/*.ailx` is written in
+- is the form every existing `examples/*.ail` is written in
 
 LLMs generate Form-A; the toolchain converts to JSON.
 
@@ -48,7 +48,7 @@ LLMs generate Form-A; the toolchain converts to JSON.
   DEF*)
 ```
 
-The module name MUST equal the file stem (`bench_list_sum.ailx` →
+The module name MUST equal the file stem (`bench_list_sum.ail` →
 `(module bench_list_sum ...)`).
 
 ## Imports
@@ -269,11 +269,11 @@ significantly.
 
 ## Few-shot corpus
 
-These four modules are real `examples/*.ailx` content. Each one is
+These four modules are real `examples/*.ail` content. Each one is
 parseable and typechecks clean. Pattern-match against them when
 generating new code.
 
-### 1 — `hello.ailx`: minimal IO program
+### 1 — `hello.ail`: minimal IO program
 
 ```
 (module hello
@@ -283,7 +283,7 @@ generating new code.
     (body (do io/print_str "Hello, AILang."))))
 ```
 
-### 2 — `borrow_own_demo.ailx`: mode annotations on a recursive list
+### 2 — `borrow_own_demo.ail`: mode annotations on a recursive list
 
 ```
 (module borrow_own_demo
@@ -333,7 +333,7 @@ generating new code.
           (do io/print_int (app sum_list xs)))))))
 ```
 
-### 3 — `lit_pat.ailx`: literal patterns and nested ctor patterns
+### 3 — `lit_pat.ail`: literal patterns and nested ctor patterns
 
 ```
 (module lit_pat