Shell layer cursor encoding/decoding for cursor-based pagination.

SIDE EFFECTS:
- JSON encoding/decoding
- Base64 encoding/decoding
- Exception throwing on invalid cursors

Cursors are opaque tokens that encode:
- Item ID (for stable ordering)
- Sort field value (for comparison)
- Sort direction (asc/desc)
- Optional timestamp (for expiry)

Format: Base64(JSON({:id ... :sort-value ... :sort-field ... :sort-direction ...}))

Shell layer RFC 5988 Link header generation for pagination.

RFC 5988: Web Linking
https://datatracker.ietf.org/doc/html/rfc5988

SIDE EFFECTS:
- String building and URL encoding
- Logging

Link headers provide hypermedia controls for pagination navigation.
They allow clients to discover next, prev, first, and last pages without
parsing response bodies.

Format:
Link: </api/v1/users?limit=20&offset=20>; rel="next",
      </api/v1/users?limit=20&offset=0>; rel="first",
      </api/v1/users?limit=20&offset=980>; rel="last"

Supported Relations:
- first: First page of results
- last: Last page of results (offset pagination only)
- prev: Previous page
- next: Next page
- self: Current page