docs(core): fix frame adapter reference links

[tianma-if]

What changed

• Replaces two stale links in packages/core/docs/core.md with existing concept docs for frame adapters and deterministic rendering.

Why

• The old links pointed at files that are no longer present in the repository, so readers landed on broken references.

Validation

• Ran a local relative-link check for packages/core/docs/core.md.

[tianma-if]

Ready for review. This PR is mergeable, kept to a small single-purpose change, and maintainer edits are enabled. Happy to adjust anything you prefer before merge.

[miguel-heygen]

Closing in the backlog value sweep. This is documentation, test-only, dependency upkeep, refactoring, or narrow diagnostic/ergonomic polish without enough current user impact or an active merge path to justify carrying it open. Please reopen or resubmit against current main if the need is still concrete.

[tianma-if]

Refreshed this fix onto current main and revalidated both replacement targets (docs/concepts/frame-adapters.mdx and docs/concepts/determinism.mdx). Formatting and git diff --check pass; CI is rerunning.

[jrusso1020]

Closing now that the refreshed, attributed successor #3667 has merged with Magi approval and passing checks. The fix was verified against current implementation and usage. Thanks for the original contribution.