Schema change: move Cache under Index

[zipdoki]

Background

A Cache is meant to back an Index — Seek looks up start keys from the cache, and Scan continues from those start keys against the index. For this handoff to stay consistent, a cache must always exist paired with the index it backs.

Today the schema lets Cache be declared on its own, separate from any Index. That makes it easy for a cache to drift out of sync with the index it's supposed to back. We want to fix this at the schema level by making Cache a child of Index, so the two can never be defined independently.

There is also a related cleanup: Seek returns a set of start keys (not a single position), so the offset field on the Seek API no longer makes sense and should be removed first.

Task

• Drop offset from the Seek API (request schema, server, clients) — done first
• Move cache under index in the schema
• Update docs and examples to the new schema shape
• Write migration notes for users with existing top-level cache entries

Done When

• The Seek API no longer accepts offset.
• An index declares its cache inline, as shown below, and Seek/Scan share the same key layout end-to-end.
• Existing tests pass; new tests cover the nested-cache path.

{
  "indices": [
    {
      "name": "permission_created_at_desc",
      "fields": [
        {
          "name": "permission",
          "order": "ASC"
        },
        {
          "name": "createdAt",
          "order": "DESC"
        }
      ],
      "desc": "createdAt desc index",
      "cache": {
        "dimension": [
          {
            "field": "permission",
            "values": [
              "others"
            ]
          }
        ],
        "limit": 100
      }
    }
  ]
}

Notes

• Callers that used offset on Seek should switch to paging with the Scan cursor.
• Existing top-level cache entries need to be moved under their owning index.

[em3s]

We're moving to the v3 DDL API, so could you rewrite this against v3? If v3 doesn't have enough to cover this requirement yet, we should update the v3 DDL first before proceeding.