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/ail/tests/e2e.rs

@@ -429,8 +429,8 @@ fn render_parse_round_trip_canonical() {
         .expect("run ail render");
     assert!(render.status.success(), "ail render failed");
 
-    let tmp = std::env::temp_dir().join(format!("ailang_render_e2e_{}.ailx", std::process::id()));
-    std::fs::write(&tmp, &render.stdout).expect("write rendered ailx");
+    let tmp = std::env::temp_dir().join(format!("ailang_render_e2e_{}.ail", std::process::id()));
+    std::fs::write(&tmp, &render.stdout).expect("write rendered ail");
 
     let parsed = Command::new(ail_bin())
         .args(["parse", tmp.to_str().unwrap()])
@@ -2153,7 +2153,7 @@ fn build_and_run_with_rc_stats(example: &str) -> (String, u64, u64, i64) {
 /// not leak the LCons outer cells.
 ///
 /// Background: 18f.2's tail-latency bench found that
-/// `bench_latency_explicit.ailx` peaks at the same RSS as the
+/// `bench_latency_explicit.ail` peaks at the same RSS as the
 /// implicit-mode variant despite carrying mode annotations
 /// throughout the hot path. Diagnosis: in
 ///