Serde and pagination: deserializeFrom abort, paginator leak on malformed PageInfo, tristate null cast, SSE double-reported release failure #79

Part of the audit remediation umbrella #67. Milestone 4. Severity: MEDIUM. Requirements: SERDE-3, SERDE-14, PAGE-2, PAGE-27, SSE-30.

Current SDK behavior

• {signal} cannot interrupt a pending read. deserializeFrom
  (packages/codec-json/src/json-serde.ts:219-225) checks the signal before the lock and after
  each chunk. An await reader.read() that never resolves is not raced against the signal. A
  stream that enqueues {"a": and stalls, with a signal aborted 20 ms later, never settles and
  leaves source.locked === true. seams/serde.ts:210-212 and
  docs/sdk-documentation/write-a-serde.md:118 promise "an aborted call never leaves the
  caller's source locked". serializeTo (serde.ts:98) has the same shape. The only tests use
  already-aborted signals; tests/node-conformance/serde.test.mjs has no {signal} or
  admitsNull case.
• A malformed PageInfo leaks the response. paginator.ts:180 clears held before
  transport.send. If parse resolves to undefined or {items: null}, the invariant at
  :203 or the Page constructor at :212 throws while held === undefined, so the finally
  at :221 closes nothing. parseOrClose (:231-250) covers parse-throws only. Both
  invariant messages (paginator.ts:207-210, page.ts:74-77) say "never null" but test
  !== undefined; items: null then throws a bare TypeError from the spread at page.ts:81.
• tristate() can produce a present null. tristate-schema.ts:45 does
  present(inner.parse(input) as NonNullable<T>). An inner schema that normalizes to null
  yields {kind: 'present', value: null}, the fourth state SERDE-14 forbids.
• A release failure during an in-flight SSE error is reported twice. sse/stream.ts:135,160-176:
  the close error is suppressed onto the thrown SuppressedError and also passed to
  onReleaseFailure. SSE-30 scopes the hook to the automatic clean-terminal path with no error
  in flight.

Expected behavior

An aborted deserializeFrom or serializeTo settles promptly and leaves the caller's stream
unlocked. The paginator closes the response on every exit path, including a strategy that
returns a malformed result, and names the invariant it violated. tristate() never returns a
present null. A release failure is reported once: on the thrown error when an error is in
flight, through onReleaseFailure otherwise.

Notes and leads

• Abort: race reader.read() against the signal (Promise.race with an abort listener, removed
  in finally), then releaseLock() as today. If the race is not wanted, narrow the doc to
  "checked between chunks" instead. Mid-drain test in json-serde.test.ts and in
  tests/node-conformance/serde.test.mjs.
• Paginator: wrap the two invariants in the same close-then-rethrow discipline as
  parseOrClose; test == null to catch both. Tests: parse → undefined, parse → {items: null},
  assert close was called once.
• tristate: runtime check after inner.parse; throw DeserializationError.
• SSE: skip the hook in #releaseWithInFlightError.
• Patch changesets for @dexpace/core and @dexpace/codec-json.