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/roadmap.md

@@ -171,7 +171,7 @@ context. Pick the next milestone from P1.)_
   type name). In the known-owner branch, list the owner's available
   type defs as candidates the way `bare-cross-module-type-ref`
   lists candidates from imports.
-  - context: fieldtest 2026-05-11 — `examples/ct_3*.ailx` exhibits both branches with identical-shape diagnostics.
+  - context: fieldtest 2026-05-11 — `examples/ct_3*.ail` exhibits both branches with identical-shape diagnostics.
 - [ ] **\[todo\]** Workspace search beyond entry-module's directory —
   `load_workspace` only finds sibling `.ail.json` files in the same
   directory as the entry module, so any consumer of prelude/std in a
@@ -182,12 +182,12 @@ context. Pick the next milestone from P1.)_
   - context: fieldtest 2026-05-11 — fieldtest fixtures could not be
     placed under `examples/fieldtest/` because of this; predates the
     canonical-type-names milestone but surfaces every time.
-- [ ] **\[todo\]** `ail check`/`build`/`run` accept `.ailx` extension —
-  today `ail check foo.ailx` produces a misleading JSON-parse error
+- [ ] **\[todo\]** `ail check`/`build`/`run` accept `.ail` extension —
+  today `ail check foo.ail` produces a misleading JSON-parse error
   (`json: expected value at line 1 column 1`) because the loader only
   accepts `.ail.json`. Either teach the CLI subcommands to auto-parse
-  `.ailx` internally, or detect the extension and emit a
-  "did you mean `ail parse foo.ailx`?" hint. The LLM-author's natural
+  `.ail` internally, or detect the extension and emit a
+  "did you mean `ail parse foo.ail`?" hint. The LLM-author's natural
   first command should not be the one that produces a misleading
   diagnostic.
   - context: fieldtest 2026-05-11 — orthogonal to canonical-type-names
@@ -205,13 +205,13 @@ context. Pick the next milestone from P1.)_
 
 ## P3 — Ideas
 
-- [ ] **\[todo\]** `compare_primitives_smoke.ailx` counterpart —
+- [ ] **\[todo\]** `compare_primitives_smoke.ail` counterpart —
   the canonical happy-path exhibit for canonical-type-names ships
   only as JSON. Per Decision 6, Surface is the LLM-author surface;
-  the milestone should have a `.ailx` counterpart. Two paths: author
-  `examples/compare_primitives_smoke.ailx` to round-trip into the
+  the milestone should have a `.ail` counterpart. Two paths: author
+  `examples/compare_primitives_smoke.ail` to round-trip into the
   existing JSON, OR retire the JSON fixture in favour of
-  `examples/ct_1_ordering_signum.{ailx,ail.json}` as the new canonical
+  `examples/ct_1_ordering_signum.{ail,ail.json}` as the new canonical
   happy-path exhibit.
   - context: fieldtest 2026-05-11 (spec_gap).
 - [ ] **\[todo\]** Codegen `lookup_ctor_in_pattern` type-anchoring —