Accept: application/x-msgpack on a collection GET returns a JSON-wrapped Buffer, not raw msgpack

`Accept: application/x-msgpack` on a **collection** GET (`GET /T/`) returns a JSON-serialized Buffer — `{"contentType":"application/msgpack","data":{"type":"Buffer","data":[...]}}` — instead of raw msgpack bytes. The `Content-Type` header says `application/x-msgpack` but the body is JSON, so a msgpack client cannot decode it.

**Root:** `resources/contentTypes.ts:221-226` — the msgpack serializer returns `pack(data)` (a Node Buffer) for non-streaming (collection) data; Fastify's default JSON serializer then JSON-stringifies that Buffer. The **CBOR** path avoids this by returning a `Readable` stream (`EncoderStream`), which Fastify pipes raw. A single-record point-GET msgpack response is also fine (different code path).

**Fix:** have the msgpack collection serializer return a stream — `Readable.from([pack(data)])` — like the CBOR path.

**Scope/clean legs:** CBOR (single + collection) works; single-record cross-format round-trips work; q-value Accept negotiation works; GraphQL is JSON-only by design.

**Severity:** medium — msgpack list endpoints are unusable; CBOR is the working binary format. Related: `application/msgpack` (without the `x-`) isn't a registered media type (use `application/x-msgpack`).

**Repro:** `npm run test:integration -- "integrationTests/qa-scratch/qa358-content-negotiation.test.ts"`

— filed by KrAIs (Claude) on Kris's behalf (qa-explorer campaign, main @ 28db4fde4 v5.1.12)

Start in resources/contentTypes.ts:221-226, where the non-streaming msgpack collection serializer returns pack(data), and compare it with the CBOR stream path. Run npm run test:integration -- "integrationTests/qa-scratch/qa358-content-negotiation.test.ts" first. Done means collection GET responses accepted as application/x-msgpack contain raw msgpack bytes that the test can decode.

Written by the indexing model from the issue text.