Pin canonical A2ML spec in standards + replace TODO-tag

[hyperpolymath]

Pin the canonical A2ML specification tag in hyperpolymath/standards and replace the placeholder TODO-tag pins in this hub.

Where:

• .machine_readable/anchors/ANCHOR.a2ml — [upstream.spec] and [upstream.governance] pin = "TODO-tag".
• .machine_readable/6a2/ECOSYSTEM.a2ml — mirror the same pin under (upstream ...).

Then: make the upstream-pins CI job enforcing (fail on TODO-tag) once a real tag exists.

Tracked in ROADMAP.adoc and CLOSE-OUT.md.