fix: correct PAIR OpenRTB identity mapping

[bokelley]

Summary

• backport the PAIR OpenRTB identity mapping correction from fix: reconcile stale billing state and correct PAIR mapping #6698 to the 3.1.x release line
• emit pair-protocol.com, preserve publisher-owner scope through EID.matcher, and treat the former AdCP source only as a legacy input alias
• clarify that PAIR wire identifiers are publisher-scoped and add the required patch changeset

Scope

This cherry-picks only the isolated PAIR protocol/docs commit from #6698. It intentionally excludes the billing reconciliation change and main's beta pre-mode metadata.

Validation

• schema validation suite: 20/20
• changeset protocol scope check
• Changesets status reports an adcontextprotocol patch
• push-time version synchronization, schema-link, and Mintlify broken-link checks

[aao-secretariat]

Ladon verdict: Approve

Approve — clean backport of the PAIR OpenRTB identity-mapping correction from #6698 to the 3.1.x line.

Checked:

• No no-auto-approve team match; gated_paths false; high_risk false.
• Reviewer surfaced no Critical/High/Medium findings.
• Enum wire values untouched (only descriptions changed) in match-id-type.json and uid-type.json.
• Publisher-scoped/EID.matcher framing consistent across all five files; OpenRTB source correction ships with an explicit legacy-input-alias transition path.
• One patch changeset present.

No findings fired rows 1–8; falls through to row 9 → approve.