Wave D docs polish: reconcile CHANGELOG cross-bullet contract + add classical restriction to api/spillover.rst

Pre-this-commit, the [Unreleased] block contained internally inconsistent
SpilloverDiD bullets: the Wave B and Wave C entries (added first) each
said the Gardner GMM correction was "NOT applied" / "planned follow-up",
while the Wave D entry (added later in the same release) shipped that
exact correction. Update the Wave B and Wave C variance subsections to
note explicitly that they are "Superseded by the Wave D Gardner GMM
first-stage correction in this same release", cross-referencing the
Wave D bullet.

Also: docs/api/spillover.rst documented the HC2/HC2_BM
NotImplementedError but omitted the new vcov_type="classical"
restriction. Add a classical bullet alongside the HC2/HC2_BM one with
the documented synthesis pointer to hc1 / conley / cluster.

Docs-only change; no source / test edits.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber