19 - adf/ is what both formats read, and a construct rises on its second consumer

AGENTS.md

@@ -349,19 +349,26 @@ someone spells it or pins it.
 - No casts: `as`, `as unknown as`, non-null `!`. A boundary owes a type guard validating the
   fields it claims (`isAdfDocument`); past it everything is typed. Make invalid states
   unrepresentable.
-- `src/adf/` holds ADF's own knowledge and imports no format. Each format directory (`markdown/`,
-  `html/`) parts into `emit/` (ADF→format) and `parse/` (format→ADF), the rest of it holding what
-  both directions read. A construct's reader lives there beside the regex the emitter escapes
-  against, so the two cannot drift; a reader with no emit counterpart goes in `parse/`, unless it is
-  part of a construct that side already holds — a grammar stays in one file rather than splitting
-  across the seam. A rule both
-  directions must answer alike — whether a list marker interrupts a paragraph — is one function
-  there too, never a copy per direction, however conservative the copy would be. Where the rule is
-  the emitter's own choice, input consults it rather than restating it: the parser asks
-  `commonMarkSpelling` which form the emitter picks, and `openingLinkTakesDirective` whether the
-  line a paragraph's opening link starts forces the directive link, so no fixture the emitter
-  writes can be refused, and a spelling the emitter refuses gives its own error rather than a
-  second name for it.
+- `src/adf/` holds ADF's own knowledge, imports no format, and is where a construct both formats
+  read lives: the question is answered in ADF's vocabulary — a node type, an attribute kind, a
+  content model — and no delimiter, element name or escape reaches it. A helper that cannot answer
+  that way is two constructs, the ADF question there and the spelling in each format, the seam
+  `markAttributes` and `markSpellings` already draw; one that cannot be split is a gap to ask (§15).
+  `markdown/` and `html/` are peers: neither imports the other, and no third directory sits between
+  them. A primitive knowing neither ADF nor a format — `result.ts`, `json-value.ts`, `nesting.ts`,
+  `canonical-json.ts` — stays at `src/` root. A construct rises to `adf/` on its second consumer,
+  not in anticipation of one (the maintainer, 2026-09-21).
+- Each format directory (`markdown/`, `html/`) parts into `emit/` (ADF→format) and `parse/`
+  (format→ADF), the rest of it holding what both directions read. A construct's reader lives there
+  beside the regex the emitter escapes against, so the two cannot drift; a reader with no emit
+  counterpart goes in `parse/`, unless it is part of a construct that side already holds — a grammar
+  stays in one file rather than splitting across the seam. A rule both directions must answer
+  alike — whether a list marker interrupts a paragraph — is one function there too, never a copy
+  per direction, however conservative the copy would be. Where the rule is the emitter's own
+  choice, input consults it rather than restating it: the parser asks `commonMarkSpelling` which
+  form the emitter picks, and `openingLinkTakesDirective` whether the line a paragraph's opening
+  link starts forces the directive link, so no fixture the emitter writes can be refused, and a
+  spelling the emitter refuses gives its own error rather than a second name for it.
 - The attribute vocabulary is ADF's: `adf/` walks it and narrows each value to its kind, and a
   format spells the narrowed value. A spelling that re-checks the type is the check's second copy.
   Reading a spelling back is the format's own: the reader sits beside the spelling it inverts, so

todo-history.md

@@ -907,6 +907,21 @@ The done `todo.md` items in full, as they were written. `todo.md` keeps a one-li
       (the stability-reviewer, 2026-09-19). The one list accounting was not
       taken: 4b settled that accounting the day this was filed, and reopening it is an ask rather
       than a chunk.
+- [x] **19 — A home for what both formats read (`0.2.0`).** Settle where a construct both formats
+      need lives, and say so in AGENTS.md §11. Today `adf/` may hold no format knowledge and each
+      format directory holds its own shared layer, so there is no third place; the first ADF-shaped
+      but format-touching helper either breaks the layering or becomes a second spelling of one
+      rule, which is the loss §2 exists to stop. Both architects ranked this first and the only
+      item cheaper before the feature than after.
+      **Settled** (the maintainer, 2026-09-21): `adf/` is that place, and the test is the vocabulary
+      the answer is in — a node type, an attribute kind, a content model, never a delimiter, an
+      element name or an escape. A helper that cannot answer that way is the ADF question there and
+      a spelling per format, the seam `markAttributes` and `markSpellings` already draw; one that
+      cannot be split is a gap to ask. `markdown/` and `html/` are peers with no third directory
+      between them, and `src/` root keeps the primitives knowing neither ADF nor a format. No code
+      moved: a construct rises on its second consumer, so `linkHref` (`markdown/mark-spellings.ts`)
+      and the external-image read (`markdown/emit/image.ts`), both pure ADF attribute reads, move
+      when `html/` reads them (7).
 - [x] **28 — `emitLine`'s retry loop cannot spin (`0.2.0`).** `emit/inline-line.ts:67` is a
       `for (;;)` that re-emits the line until every unspellable node has been carried, and its
       termination rests on a comment: each pass carries at least one more node, or flips

todo.md

@@ -42,12 +42,6 @@ panel says the next reader pays for.
 29 and 30 come from 17's prose pass (2026-09-20). 29 reads first because every goal is what a later
 ask is settled against, 19's included; 30 sits beside 25, the other chunk rereading AGENTS.md.
 
-- [ ] **19 — A home for what both formats read (`0.2.0`).** Settle where a construct both formats
-      need lives, and say so in AGENTS.md §11. Today `adf/` may hold no format knowledge and each
-      format directory holds its own shared layer, so there is no third place; the first ADF-shaped
-      but format-touching helper either breaks the layering or becomes a second spelling of one
-      rule, which is the loss §2 exists to stop. Both architects ranked this first and the only
-      item cheaper before the feature than after.
 - [ ] **20 — The give-way channel is unmistakable (`0.2.0`).** `emitBlockquote`, `emitCodeBlock`,
       `emitHeading`, `emitList`, `emitParagraph` and `emitRule` return
       `Result<EmittedBlock> | undefined`, where `undefined` gives way to the directive form and an
@@ -285,6 +279,7 @@ ask is settled against, 19's included; 30 sits beside 25, the other chunk reread
 - [x] **16 — The link wrapping a link (`0.2.0`).**
 - [x] **17 — A machine-enforced size ratchet (`0.2.0`).**
 - [x] **18 — The subtree the directive spelling asks about (`0.2.0`).**
+- [x] **19 — A home for what both formats read (`0.2.0`).**
 - [x] **28 — `emitLine`'s retry loop cannot spin (`0.2.0`).**
 - [x] **29 — The README reads raw HTML as refused for good (`0.2.0`).**
 - [x] **30 — AGENTS.md says each thing once (`0.2.0`).**