Pin refusal codes and divergences; fix the text oracle and count by heading level

corpus/README.md

@@ -12,9 +12,13 @@ One directory per contract kind, each landing with its milestone:
   pins which error.
 - `real-payloads/` — `<name>.json`: sanitized live ADF, round-tripped ADF→markdown→ADF. No
   expected markdown.
-- `commonmark-spec/` — `spec.json` (the CommonMark 0.31.2 suite, vendored byte-exact) and
-  `exceptions.json` (the examples a known divergence exempts, keyed by `check` and `example` with
-  a `reason`).
+- `commonmark-spec/` — the CommonMark suite run against `markdownToAdf` by three checks. `spec.json`
+  (0.31.2, vendored byte-exact) is the suite; `refusals.json` pins each refusing example to its
+  error `code`; `exceptions.json` pins each known divergence by `check`, `example`, `kind` and the
+  exact `divergence`, with a `reason`. `kind` is `mark-model` (the permanent count divergence from
+  ADF's mark-per-text-node model), `unspellable` (parses but the flavour has no spelling) or
+  `pending` (a parser gap a later milestone may close). The two lists are derived and labelled,
+  not hand-picked — regenerate them rather than hand-edit when re-pinning.
 
 JSON is editor-normal (AGENTS.md §2), two-space indent, keys sorted. `spec.json` is the vendored,
 upstream machine-readable suite (CommonMark 0.31.2, CC-BY-SA-4.0, © John MacFarlane) and is not