design: ratify form (B) prose projection (iter 20e tidy)

Family-20 tidy-iter: ailang-architect drift review surfaced
DESIGN.md silence on the new prose surface as the highest-leverage
gap. Decision 6 now has a 'Form (B) — human prose projection'
subsection naming the lossy-vs-lossless contract, the no-parser-
by-design choice, and the LLM-mediated round-trip. CLI block
gained 'ail prose' and 'ail merge-prose'.

No code changes. Form (B) does not weaken any Decision 6
invariant: JSON-AST remains the only hashable artefact, form (A)
the canonical authoring surface, 30-production grammar unchanged.

Two architect findings deferred (prompt-template lockstep test;
snapshot-fixture cross-family coupling). Family 20 closes.
JOURNAL queue: empty.

docs/JOURNAL.md

@@ -9167,3 +9167,80 @@ practice the family stays as-shipped.
     validates the merge product; `merge-prose` only composes the
     prompt.
 
+
+## 2026-05-08 — Family-20 tidy-iter (Iter 20e)
+
+Per CLAUDE.md "Tidy-iter at family boundaries" — every named iter
+family closes with a tidy-iter. `ailang-architect` ran a drift
+review of family 20 and reported three actionable items, plus two
+non-actionable observations.
+
+### Architect findings
+
+  1. **DESIGN.md silent on form (B).** Decision 6 documents form (A)
+     extensively but never names the new prose surface. The CLI
+     section (`docs/DESIGN.md` "## CLI") lists `render`/`parse` but
+     not `prose`/`merge-prose`. JOURNAL has the design pinning, but
+     DESIGN.md is the canonical spec — this is the highest-leverage
+     fix.
+  2. **Prompt-template drift hazard.** `compose_merge_prose_prompt`
+     in `crates/ail/src/main.rs` and the prompt block in
+     `docs/PROSE_ROUNDTRIP.md` are byte-equivalent twins maintained
+     by hand. No test enforces equality. Bounded — the comment in
+     `main.rs` line 240–242 already names the lockstep — but real.
+  3. **Snapshot-fixture coupling.** `ailang-prose/tests/snapshot.rs`
+     binds prose tests to four 18-family RC fixtures. Any future
+     iter touching those examples must re-render the `.prose.txt`.
+     Not actionable until a real refactor forces it.
+
+### Resolved this iter
+
+**Item 1 — ratified.** Two edits to `docs/DESIGN.md`:
+
+  - New subsection at the end of Decision 6 (after Iter 14e
+    refinements, before Decision 7): `### Form (B) — human prose
+    projection (Family 20)`. ~50 lines documenting form (B) as the
+    "display" projection Decision 6's architectural pin already
+    anticipated, the lossy-vs-lossless contract, and the fact that
+    form (B) has no parser by design (round-trip is LLM-mediated,
+    not compiler-passed).
+  - CLI block extended with `ail prose` and `ail merge-prose` lines.
+
+The new subsection explicitly does NOT weaken any Decision 6
+invariant: JSON-AST remains the only hashable artefact, form (A)
+remains the canonical authoring surface, the 30-production grammar
+is unchanged, and `check`/`codegen` stay projection-agnostic.
+
+### Deferred
+
+**Item 2 (prompt-template lockstep)** — the existing
+`// Keep this template in lockstep with docs/PROSE_ROUNDTRIP.md`
+comment at `main.rs:240–242` is the audit trail. Adding an
+equality test would require parsing the Markdown block in the
+doc; brittle. Stays queued; will close if the next iter touching
+the prompt actually surfaces the drift.
+
+**Item 3 (snapshot-fixture coupling)** — not actionable; documented
+as known consequence of cross-family snapshot binding.
+
+### Family 20 final state
+
+- 20a: prose renderer skeleton + `ail prose` CLI
+- 20b: infix + paren elision + doc-wrap + nested-match
+- 20c: rolled into 20a
+- 20d: `ail merge-prose` + `docs/PROSE_ROUNDTRIP.md`
+- 20e: this tidy-iter — DESIGN.md ratification
+
+Three deferred polishes from 20b (let-inlining, `do io/print_int`
+→ `print` sugar, doc-wrap widow control) remain queued; all three
+need real-corpus signal to know whether the heuristic is right.
+
+JOURNAL queue: empty again.
+
+### Files touched this iter
+
+- `docs/DESIGN.md` — Form (B) subsection in Decision 6, CLI block
+  extension. ~60 lines added.
+- `docs/JOURNAL.md` — this entry.
+
+No code changes. Build/test status unchanged from 20d (green).