ail: merge-prose subcommand + PROSE_ROUNDTRIP.md (iter 20d)

Closes family 20: prose-edit cycle is end-to-end. ail merge-prose
<original.ail.json> <edited.prose.txt> composes a shaped LLM prompt
to stdout. User pipes to their LLM (Claude/etc.), gets new .ail.json,
runs ail check, saves.

No built-in API client — AILang stays a compiler + tooling, the LLM
mediator is supplied externally. PROSE_ROUNDTRIP.md documents the
6-step cycle, the prompt contract (preserve modes/effects/tail
flags from original), failure modes, and the design choice to omit
an API client.

Family 20 now closes: 20a renderer + 20b polish + 20d mediator
shipped; 20c rolled into 20a. Three deferred polishes (let-inlining,
print sugar, doc-wrap widow control) wait for corpus signal.

crates/ail/tests/e2e.rs

@@ -2372,3 +2372,51 @@ fn alloc_rc_partial_drop_at_app_let_close_does_not_leak_unmoved_fields() {
          `partial_drop_<m>_<T>(p, mask)`"
     );
 }
+
+/// Iter 20d: `ail merge-prose` reads two files and prints a prompt
+/// that frames the prose-round-trip task for an external LLM. Smoke
+/// test: writes two temp inputs, invokes the binary, asserts exit 0
+/// and that the captured stdout carries the role-statement landmark
+/// plus both inputs verbatim.
+#[test]
+fn merge_prose_prints_framed_prompt() {
+    let tmp = std::env::temp_dir().join(format!(
+        "ailang_e2e_merge_prose_{}",
+        std::process::id()
+    ));
+    std::fs::create_dir_all(&tmp).unwrap();
+    let orig_path = tmp.join("orig.ail.json");
+    let prose_path = tmp.join("edited.prose.txt");
+    let orig_bytes =
+        b"{\"schema\":\"ailang/v0\",\"name\":\"foo\",\"imports\":[],\"defs\":[]}";
+    let prose_bytes = b"// module foo\n\nfn bar() -> Int { 42 }\n";
+    std::fs::write(&orig_path, orig_bytes).unwrap();
+    std::fs::write(&prose_path, prose_bytes).unwrap();
+
+    let output = Command::new(ail_bin())
+        .args(["merge-prose"])
+        .arg(&orig_path)
+        .arg(&prose_path)
+        .output()
+        .expect("ail merge-prose failed to run");
+    assert!(
+        output.status.success(),
+        "ail merge-prose exited non-zero: stderr={}",
+        String::from_utf8_lossy(&output.stderr)
+    );
+    let stdout = String::from_utf8(output.stdout).expect("stdout utf8");
+    assert!(!stdout.is_empty(), "empty stdout from merge-prose");
+    assert!(
+        stdout.contains("integrating prose edits"),
+        "prompt missing role-statement landmark; got:\n{stdout}"
+    );
+    // Both inputs must round-trip into the prompt verbatim.
+    assert!(
+        stdout.contains("ailang/v0"),
+        "prompt missing original .ail.json content"
+    );
+    assert!(
+        stdout.contains("fn bar() -> Int { 42 }"),
+        "prompt missing edited prose content"
+    );
+}