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.

bench/reference/compute_collatz.c

@@ -1,6 +1,6 @@
 // Hand-C reference for bench_compute_collatz.
 //
-// Same algorithm as examples/bench_compute_collatz.ailx — for each
+// Same algorithm as examples/bench_compute_collatz.ail — for each
 // starting value in [1..N], count Collatz steps to reach 1, sum.
 // Three sizes: 10k / 100k / 500k starting values.
 //

bench/reference/compute_intsum.c

@@ -1,6 +1,6 @@
 // Hand-C reference for bench_compute_intsum.
 //
-// Same algorithm as examples/bench_compute_intsum.ailx — accumulate
+// Same algorithm as examples/bench_compute_intsum.ail — accumulate
 // `i * 7` for i in [n, n-1, ..., 1], printing the final acc.
 // Three sizes: 1M / 10M / 50M iterations.
 //

bench/reference/list_sum.c

@@ -1,6 +1,6 @@
 // Hand-C reference for bench_list_sum.
 //
-// Same algorithm as examples/bench_list_sum.ailx — build a linked list
+// Same algorithm as examples/bench_list_sum.ail — build a linked list
 // of [0, 1, ..., N-1] via prepending, then sum by linear traversal.
 // Three workload sizes: 100k / 1M / 3M cells, matching the AILang
 // fixture exactly so the AILang/C wall-time ratio is fair.

bench/reference/list_sum_explicit_free.c

@@ -2,7 +2,7 @@
 //
 // Companion to list_sum.c (which leaks); this version adds explicit
 // `free()` calls to mirror AILang's explicit-mode RC dec-tax. Pairs
-// with examples/bench_list_sum_explicit.ailx via bench/cross_lang.py.
+// with examples/bench_list_sum_explicit.ail via bench/cross_lang.py.
 //
 // The free pass walks the list after sum, freeing each cell. This is
 // genuinely O(N) work that bench_list_sum.c never paid; the rc/c

bench/reference/tree_walk.c

@@ -1,6 +1,6 @@
 // Hand-C reference for bench_tree_walk.
 //
-// Same algorithm as examples/bench_tree_walk.ailx — build a balanced
+// Same algorithm as examples/bench_tree_walk.ail — build a balanced
 // binary tree of given depth (every value = 1) and sum every node.
 // Three depths: 16 / 18 / 20 (= 65535 / 262143 / 1048575 nodes).
 //