fix(compilers/openapi): preserve keywords no lowering reads

[OmarAlJarrah]

Summary

A schema keyword that the lowering elected for a position never reads was
dropped: no IR field, no Unmodeled entry, no diagnostic. Two shapes of the
same bug:

• Beside a $ref.format, enum, const, required and
  additionalProperties written next to a $ref vanished at every position —
  component, property, parameter, header, allOf branch. In JSON Schema 2020-12,
  and so in OpenAPI 3.1, $ref is an ordinary keyword and its siblings are
  conjoined with it, so this is real data.
• Beside an elected keyword family.type: string next to an allOf that
  composed a Model, format and a bound next to a const that hoisted a
  Literal, and so on.

Both had one root cause. The "what did the winner not consume?" census
(preserveUnhomedKeywords) had exactly one call site, inside lower() on the
body path, so the $ref path ran no census at all; and the census that did run
walked a hand-maintained list of six shape applicators plus a special case for
format, so anything outside that list was dropped whatever it was written
beside.

The census now asks the node the position actually lowered to whether it has a
field for each keyword — a Model has no type token, a Literal has no encoding
and no Constraints, an alias over a $ref target has none of them — instead
of consulting a list of keywords worth keeping. It runs at every position a
schema is lowered at, including the three $ref sites (refSiteRef, an allOf
branch, and the annotation.HomeCarrier positions, which keep the keyword on
the carrier rather than hoisting a node). Anything homeless is kept verbatim
under Unmodeled with ReasonDegradedLowering and one info diagnostic naming
what the position lowered to.

Deliberately unchanged:

• allOf beside type: object records nothing, because the composed Model does
  assert object. That is the case a keyword list cannot decide and the node
  can, and it is asserted as a control.
• An allOf branch's required is left out of the branch census:
  applyCompositionRequired reads it, so recording it would report one keyword
  twice.
• valueConstraintKeywords is now the single list behind both
  declaresValueConstraints and the recorder, replacing a second hand-written
  copy that read model fields where the predicate read raw nodes.

A bound written beside a co-declared oneOf is still dropped; that is a
different mechanism (the alias is interned with nil constraints) and is filed as
#347.

Test plan

• Table-driven tests over both positions and both shapes:
  TestRefSiteKeywords_KeptAtEveryPosition covers all five $ref siblings at
  the component and property positions;
  TestUnhomedKeywords_ElectedLoweringKeepsWhatItCannotRead covers the elected-
  family cases and asserts the kept set whole, so keeping too much fails as
  loudly as keeping too little. Controls assert zero entries and zero
  diagnostics where nothing is lost.
• Carrier coverage: TestParams_RefSiteKeywordsAreKeptOnTheParameter,
  TestHeaders_RefSiteKeywordsAreKeptOnTheHeader,
  TestRefSiteKeywords_AllOfBranchKeepsWhatTheAliasCannotHold.
• TestKeywordHome_EveryCensusKeywordHasANodeThatCarriesIt guards the one
  agreement the census still has to keep by hand — a keyword listed with no arm
  answers "homeless" everywhere, which is exactly what a $ref site expects, so
  no ref-site test could see it. Verified by planting an armless keyword.
• Conformance corpus: testdata/conformance/openapi/unhomed-keywords.yaml gains
  the $ref-site and elected-family cases, with the aliases declared above
  their targets so the two-order oracle exercises both interning orders.
  Verified that deleting a case from the fixture reddens the suite.
• Every new row was confirmed RED with the production change reverted and the
  tests kept.
• Full gate green: gofmt, go vet, golangci-lint, go build, and the
  coverage gate at exactly 100%.

Closes#268
Closes#283