Rebuild A2ML coordination hub

[hyperpolymath]

Summary

Rebuilds hyperpolymath/a2ml-ecosystem as a coordination hub satellite of hyperpolymath/standards.

• Pins upstream spec and governance authority in ANCHOR.a2ml with TODO-tag placeholders.
• Adds ECOSYSTEM.a2ml membership manifest, 12 member submodules, spec pointer, setup notes, and anchor drift CI.
• Adds A2ML conformance fixtures: 4 valid cases, 5 invalid cases, and a manifest.

Validation

• scripts/check-membership.sh
• strict A2ML governance validation with conformance/ and members/ ignored
• strict positive A2ML fixture validation for conformance/valid
• per-file strict negative validation for all 5 invalid A2ML fixtures
• bash -n for helper scripts
• git diff --cached --check before commit