docs(versioning): add schema-publication-at-merge policy

[bokelley]

Closes #2366

Adds a ### Schema publication at merge subsection to docs/reference/versioning.mdx, placed immediately after the existing ### Breaking-change notice section. The section codifies the invariant: the latest published tag and source HEAD must always reflect the same required-field set, and any PR that changes a required array, a discriminator const, or tightens a validation constraint must be accompanied by a new tag cut before the change propagates to evaluators.

The root case (source↔published drift from PR #2315 on the rc.3 tag) is resolved by the 3.0 GA tag. This doc addition is the durable Evergreen fix: it names the invariant, explains why the drift matters for pinned-tag and alias-resolving implementors alike, covers RC cycles explicitly, and describes manual responsibility until a CI check enforces the rule.

Non-breaking justification: adds a new subsection to an existing docs page; no schema changes, no wire changes, no API surface affected.

Pre-PR review:

• code-reviewer: approved — no blockers; nits on wording and changeset style noted, addressed in revision (reviewer trigger condition changed from commit-prefix to actual schema-change categories; "staged or already cut" → "ready to publish"; redundant RC-cycle sentence removed)
• docs-expert: two blockers on initial draft: (1) conflict with the 3.x stability table's deprecation-first rule for optional→required; (2) "Why" block incorrectly scoped to alias users only. Both fixed: added "in addition to any upstream obligations" clause; broadened "Why" to cover pinned-tag implementors; added cross-link to /docs/building/schemas-and-sdks#schema-versioning.

Triage-managed PR. This bot does not currently iterate on
review comments or PR conversation threads (only on the source
issue). To unblock:

• Push fixup commits directly: gh pr checkout <num> →
  fix → push.
• Or re-trigger: comment /triage execute on the source
  issue.

See #3121
for context.

Session: https://claude.ai/code/session_01PKwE7MvNkQjTBaWssihqjj

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