design: 20-family tidy-iter — refresh Data model + ecosystem inventory

Architect drift review after Family 20 surfaced four items; (1) and
(2) are doc drift fixed here, (3) and (4) are recorded as acceptable.

DESIGN.md changes:

  - Data model section rewritten from "Data model (MVP)" claiming
    only fn/const exist to "Data model" reflecting all three Def
    kinds. Added the missing schema fields the architect named:
    `tail` flag on app/do, the `seq` / `clone` / `reuse-as` / `letrec`
    terms, `paramModes` / `retMode` on Type::Fn, `suppress` on FnDef,
    `drop-iterative` on TypeDef, `args` on Type::Con, `vars` on
    TypeDef. Each additive field's hash-stable serialisation is
    called out once at the top so individual mentions stay terse.
    `Suppress`, `ParamMode`, `Literal`, `Pattern` get their own
    sub-blocks.
  - Pipeline diagram now shows `--alloc=gc` (links libgc;
    transitional) and `--alloc=rc` (emits ailang_rc_inc / _dec;
    canonical) as two backends sharing the same MIR.
  - Ecosystem inventory gained a "Surface forms" bullet for
    ailang-surface (Iter 14c, lossless Form-A printer/parser) and
    ailang-prose (Family 20, lossy Form-B projection). CLI bullet
    now lists parse, render, prose, merge-prose.

JOURNAL entry records the architect findings, the resolutions for
items (1) and (2), and the acceptable-drift status of (3)
FnDef::synthetic factor-out (defer to next schema-additive
FnDef field) and (4) linearity.rs spurious warning (not
corpus-triggered).

Pure documentation iter — no code changes. 288 workspace tests
unchanged and green.

docs/JOURNAL.md

@@ -9638,6 +9638,129 @@ that always works without a tool-use-capable client.
 ### Family / arc state
 
 20f closes the prose-roundtrip arc for now. JOURNAL queue is
-empty again. Tidy iter for 20a / b / d / e / f together can wait
-until the next family closes — the architect drift-review
-discipline applies as before.
+empty again. The 20-family tidy-iter follows immediately per
+CLAUDE.md ("the tidy-iter is non-optional, every named family
+closes with one"); the previous note here ("can wait") was
+incorrect and has been superseded by the actual tidy below.
+
+## 2026-05-08 — 20-family tidy-iter (DESIGN.md refresh)
+
+### Why
+
+Per CLAUDE.md, every named iter family closes with a tidy-iter:
+run `ailang-architect`, resolve each item (fix the drift, ratify
+in DESIGN.md, or record in JOURNAL why it is acceptable). Family
+20 — 20a, 20b, 20d, 20e, 20f — added a whole new surface
+(`ailang-prose`), a CLI subcommand pair (`ail prose` /
+`ail merge-prose`), an embedded Form-A spec, and the drift-test
+trip-wire. The architect surfaced four items.
+
+### Architect findings (verbatim, abridged)
+
+  1. **`docs/DESIGN.md` "Data model (MVP)" block stale** —
+     missing: `tail` flag on `app`/`do`, `seq`, `clone`,
+     `reuse-as`, `let-rec`, `paramModes`/`retMode` on `fn`,
+     `suppress` on `FnDef`, `drop_iterative` on `TypeDef`,
+     `Type::Con.args`, `TypeDef.vars`. Pipeline diagram still
+     said "links libgc" only — did not mention `--alloc=rc`.
+     The deferral was already noted in 19a-arc tidy.
+  2. **Project ecosystem inventory missing `ailang-surface` and
+     `ailang-prose`** — the section's own rule says new tools
+     are added "as soon as they are established"; both crates
+     are well past that threshold (14c and 20a respectively).
+     `ail prose` and `ail merge-prose` were also missing from
+     the CLI bullet.
+  3. **`FnDef { suppress: vec![] }` synthesis fan-out at 45
+     sites** — `ailang-codegen` plus `ailang-core::desugar`
+     alone has 16. 19a-arc tidy noted 7 and deferred. Each
+     schema-additive `FnDef` / `Type::Fn` field hits every site;
+     a `FnDef::synthetic(...)` constructor is the absorbing fix.
+  4. **`linearity.rs` known debt (L117–125)** — deeply-nested
+     match on a sub-binder produces a spurious
+     `over-strict-mode` warning. Not fired by current corpus;
+     module doc records the gap. Acceptable.
+
+### Resolutions
+
+**(1) and (2)** — fixed in this tidy. `docs/DESIGN.md`:
+
+  - **Data model section rewritten end-to-end.** Was titled
+    "Data model (MVP)" with stale `MVP only fn and const`; now
+    "Data model" reflecting that all three kinds (`fn`, `const`,
+    `type`) are real surface forms. Added every additive field
+    documented in `crates/ailang-core/src/ast.rs` with the
+    schema-additive `skip_serializing_if` rationale called out
+    once at the top so each individual mention can stay terse.
+    The `Suppress`, `ParamMode`, `Literal`, and `Pattern`
+    sub-schemas now have their own blocks.
+  - **Pipeline diagram extended.** Now shows `--alloc=gc` (links
+    libgc; transitional) and `--alloc=rc` (emits
+    `ailang_rc_inc` / `_dec`; canonical) as two backends sharing
+    the same MIR. The 18a–18d additions (`Term::Clone`,
+    `Term::ReuseAs`) get a sentence pointing at where they
+    materialise under `--alloc=rc`.
+  - **Ecosystem inventory** gained a "Surface forms" bullet
+    between Language core and CLI, naming `ailang-surface`
+    (Iter 14c) as the lossless Form-A printer/parser fixed by
+    Decision 6 with `parse ∘ print = id` as gating contract,
+    and `ailang-prose` (Family 20) as the lossy Form-B
+    projection with no parser whose re-integration goes through
+    `docs/PROSE_ROUNDTRIP.md`. The CLI bullet now lists
+    `parse`, `render`, `prose`, `merge-prose`.
+
+**(3) `FnDef::synthetic(...)` constructor** — *deferred, not
+addressed in this tidy.* Reason: the right factor-out shape is
+not yet obvious. Most existing call sites differ on more than
+just `suppress` — they have to compute `params`, build a
+`Type::Fn`, etc. — so a one-arg constructor saves nothing; the
+useful constructor is roughly `FnDef::new(name, ty, params,
+body)` with `doc: None, suppress: vec![]` defaults. That is a
+real change in 45 sites and should land as its own iter when the
+schema next grows a `FnDef` field (forcing the touch anyway).
+Recorded in the JOURNAL queue.
+
+**(4) `linearity.rs` sub-binder spurious warning** —
+*acceptable, recorded.* The warning fires on a synthetic shape
+that the current corpus does not produce; module doc names the
+gap; an actual customer-impact trigger would warrant a fix, not
+preemptive work. Status quo holds.
+
+### Acceptable drift summary
+
+- `FnDef::synthetic(...)` factor-out — deferred until the next
+  schema-additive `FnDef` field (rationale: existing fan-out
+  pattern works, change is large but mechanical, and the cost
+  is amortised by piggybacking on the next forced touch).
+- `linearity.rs` deeply-nested-match-on-sub-binder spurious
+  warning — accepted (not corpus-triggered; cost of fix
+  exceeds cost of the false positive at this scale).
+
+### What this tidy does NOT do
+
+- No code changes. Pure documentation iter.
+- No new tests. Existing 288 workspace tests stay green.
+- No DESIGN.md content removed. The pre-tidy "Data model (MVP)"
+  block is replaced rather than amended because it had grown
+  factually incorrect; the historical context lives in
+  `git log` and in the iter-by-iter JOURNAL entries that
+  introduced each schema field.
+
+### JOURNAL queue (post-tidy)
+
+- **`FnDef::synthetic(...)` factor-out** — to be done when the
+  next schema-additive `FnDef` field forces the touch anyway.
+- **Deferred richer integration paths** (from 20f): tool-use
+  schemas, MCP server, LSP. All additive over the static-prompt
+  round-trip; pick when prioritised.
+- **Boehm retirement** — `--alloc=gc` is transitional;
+  `--alloc=rc` is canonical. Boehm-side scaffolding to be
+  removed once the rc backend covers the full corpus.
+- **Family 21+** (typeclasses, polymorphic ADTs at runtime,
+  pattern-binding generalisation) — long-horizon language work.
+
+### Family / arc state
+
+The 20-family is now formally closed with this tidy. Codebase
+is in good form: no architecturally load-bearing drift, all
+tests green, two minor items recorded as acceptable. The next
+iter can be a feature pick from the queue.