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/PROSE_ROUNDTRIP.md

@@ -19,14 +19,14 @@ modes to watch for.
 ## What the LLM produces
 
 Iter 20f revised a load-bearing piece of this cycle: the LLM emits
-**Form-A** (an `.ailx` document — the canonical authoring surface
+**Form-A** (an `.ail` document — the canonical authoring surface
 fixed by Decision 6), not JSON-AST. JSON-AST is the canonical
 hashable artefact, but it is not a writing surface — every type
 reference wraps in `{"k": "con", ...}`, every term in
 `{"t": "...", ...}`, and a single missing `param_modes` entry is a
 schema error rather than a parse error with a position.
 
-Form-A is the form `examples/*.ailx` are written in, the form
+Form-A is the form `examples/*.ail` are written in, the form
 `ail render` produces, and the form `ail parse` consumes. The full
 LLM-targeted specification is shipped inside the binary as
 `ailang_core::FORM_A_SPEC` (sourced from
@@ -39,8 +39,8 @@ LLM-targeted specification is shipped inside the binary as
 1.  ail prose foo.ail.json > foo.prose.txt
 2.  $EDITOR foo.prose.txt                     # human edits freely
 3.  ail merge-prose foo.ail.json foo.prose.txt > prompt.txt
-4.  cat prompt.txt | <your-llm-cli> > foo.new.ailx
-5.  ail parse foo.new.ailx > foo.new.ail.json
+4.  cat prompt.txt | <your-llm-cli> > foo.new.ail
+5.  ail parse foo.new.ail > foo.new.ail.json
 6.  ail check foo.new.ail.json
 7.  mv foo.new.ail.json foo.ail.json          # if check is clean
 ```
@@ -74,7 +74,7 @@ CLI helper.
 You are integrating prose edits back into an AILang module.
 
 ROLE
-Your job is to produce an updated AILang module in Form-A (an .ailx
+Your job is to produce an updated AILang module in Form-A (an .ail
 file) that reflects the human's prose edits while preserving the
 load-bearing semantic detail from the original module.