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/src/main.rs

@@ -190,7 +190,7 @@ enum Cmd {
         #[arg(long)]
         json: bool,
     },
-    /// Parses a `.ailx` source file (form (A)) into canonical
+    /// Parses a `.ail` source file (form (A)) into canonical
     /// `.ail.json`. Iter 14c addition; symmetric to `render`.
     ///
     /// The form-(A) projection is one of potentially many producers of
@@ -269,7 +269,7 @@ fn compose_merge_prose_prompt(original_form_a: &str, edited_prose: &str) -> Stri
         "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.
 
@@ -826,7 +826,7 @@ fn main() -> Result<()> {
             }
         }
         Cmd::Parse { path, output } => {
-            // Read .ailx, parse via the surface crate, emit canonical
+            // Read .ail, parse via the surface crate, emit canonical
             // JSON. Symmetric to `render` (which goes the other way).
             let src = std::fs::read_to_string(&path)
                 .with_context(|| format!("reading {}", path.display()))?;