feat(okf-v0.2): D4 — the Attested Computation contract, format only

Upstream §10 adds a concept type carrying a sanctioned way to compute a
value. This lands its FORMAT: the five contract fields for emission and
round-trip, and §10.2's one requirement. No execution — upstream defers
the receipt and verdict wire formats, so there is nothing to build a
runtime against.

Two additions, both additive:

- The five fields (`runtime`, `parameters`, `computation`, `executor`,
  `attester`) join `OKF_V0_2`'s emission order as one block, internally
  in §10.2's own listing order. Without it they still emit — in `emit`'s
  sorted tail, where `attester` precedes `runtime`, alphabetical order
  standing in for the contract's own. No bundle that carries none of the
  keys changes by a byte, and the v0.1 profiles gain nothing.

- `FrontmatterSchema.required_by_type` expresses "`runtime` is REQUIRED
  for this type and no other" — the first rule here keyed off a
  frontmatter VALUE rather than a key. It cannot be `required`, which
  would demand `runtime` of every document. A type the mapping does not
  name carries no extra requirement, which is what keeps it inside §14:
  a consumer must not reject on an unknown `type`, so a conditional
  keyed on an unknown type stays silent rather than guesses.

Also pinned, measured today: the line-oriented parser cannot read §10's
canonical BLOCK form. `executor` and `attester` both carry a `resource`,
and with no indentation model the second overwrites the first —
`executor.resource` is lost silently, no error. Characterized rather
than fixed: reading that form needs the structured reader (D1b), and a
half-reader that drops half a contract is worse than one that never
claimed to read it. CLAUDE.md gains the invariant that falls out of it:
we emit flow form, never block, or we write bundles we cannot read back.

578 tests, mypy --strict clean, goldens byte-identical.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KKKMwi7e7PVHoFW6dJK5XP

CLAUDE.md

@@ -106,6 +106,15 @@ supported and the unspecified runtime is not; it re-enters scope when upstream
 specifies it. Because "always latest" decays silently, the release checklist
 carries an upstream-version re-check.
 
+**Structured frontmatter values are emitted in YAML *flow* form, never block.**
+Both are valid YAML and an upstream reader recovers the same structure from
+either, but this library's parser is line-oriented: it round-trips a flow
+mapping as an opaque value and cannot read the block form at all — two block
+mappings sharing an inner key (§10.2's `executor` and `attester`, both carrying
+`resource`) collapse into one namespace and the first is lost silently.
+Emitting block would produce bundles we cannot read back. Reading it needs the
+structured reader (D1b); until then the constraint binds what we write.
+
 **Every upstream release runs `docs/upstream-okf-upgrade-runbook.md`.** Pin the
 commit, enumerate the whole `okf/` tree, **read the shipped example bundles and not
 only `SPEC.md`**, classify the diff, measure our exposure and each consumer's, plan