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.

docs/journals/INDEX.md

@@ -28,3 +28,4 @@
 - 2026-05-12 — iter ms.1: pipeline anyhow-chain preservation — `{e}` → `{e:#}` at `pipeline.rs:118` restores `serde_json` parse-error leaf in JSON-cohort feedback; pinning unit test added (14/14 green) → 2026-05-12-iter-ms.1.md
 - 2026-05-12 — iter ms.2: Qwen3-Coder-Next retroactive re-run + first CodeLlama-13b-Instruct run via IONOS; DESIGN.md §Decision-6 addendum extended from 2-col single-subject to 4-col two-subject; both subjects agree on direction (AILX cheaper + ≥ green); roadmap P3 multi-subject entry removed → 2026-05-12-iter-ms.2.md
 - 2026-05-12 — audit-ms: milestone close (Multi-subject Authoring-Form Test — CodeLlama Replication) — architect clean, bench all-green (same 5-metric latency.explicit_at_rc improvement cluster as audit-cma earlier today, baseline left pristine for second consecutive audit) → 2026-05-12-audit-ms.md
+- 2026-05-12 — iter ext-rename: `.ailx` → `.ail` extension rename across the live toolchain (61 example renames + 35 content edits + experiment-crate cohort rename `ailx → ail`); historical docs (journals, archive, specs, plans, frozen experiment runs) deliberately untouched; cargo+bench all-green; opens follow-up `.ail`-CLI-acceptance iter → 2026-05-12-iter-ext-rename.md