refactor: build manifestTypes from named media-type constants (de-duplicate literals)

[m-ferrero]

Problem

The manifest content-negotiation accept-list manifestTypes (src/registry/http.ts) is built from bare string literals, but named constants for two of those exact media types already exist and are bypassed:

• application/vnd.docker.distribution.manifest.list.v2+json — already held by dockerManifestListContentType in src/manifest.ts (and used there to validate a manifest's mediaType).
• application/vnd.oci.image.index.v1+json — already held by ociImageIndexContentType in src/registry/r2.ts, which is even imported into http.ts and used for index handling — yet the accept-list re-types the literal instead of using it.
• application/vnd.oci.image.manifest.v1+json has no named constant at all.

These strings are the OCI/Docker content-negotiation contract and must stay byte-identical to the spec. With the same value living in two places, editing one (say, as the spec evolves) silently diverges from the other: the index-handling path would use the updated constant while the accept-list kept the stale literal, so a client sending that type would be rejected at negotiation even though the handler was ready for it — a failure a happy-path test using one consistent string would not catch.

Solution

Add a single src/media-types.ts (a zero-import leaf module) that defines every manifest media-type constant exactly once, and build manifestTypes from those constants. The existing consumers — src/manifest.ts, src/registry/r2.ts, src/registry/http.ts, src/router.ts — all import from that one source.

Pure, behavior-preserving refactor: the string values and the array order are unchanged (order matters — Accept is emitted as manifestTypes.join(", ")), as const is kept so the ManifestType union resolves to the identical set, and no runtime behavior changes.

Tests

Adds test/media-types.test.ts asserting manifestTypes holds the expected set in the expected order, plus each constant's value.

Part of #134.