feat: make active external claims expire without refreshed state - #147

Why

CounterProof now binds public claims to evidence, but an external relationship can still change after the evidence bytes remain valid.

An "active interoperability" statement is especially vulnerable: the handoff may remain reproducible while the upstream issue/PR state changes underneath it.

What changed

• add external-state.yml with dated observations and refresh budgets
• bind the AVERA interoperability public claim to its current upstream issue/PR state
• make active external claims fail CI after their freshness budget expires
• record the current AVERA state explicitly: issue fix: discover Node ESM/CommonJS regression tests #12 closed, implementation PR fix: surface changed test-support as proof-integrity risk #14 merged
• add an interoperability note for the in-toto conformance-layer node (#554 / #597)
• separate byte-valid provenance from current external-state applicability

This intentionally turns freshness into a maintenance obligation: a public claim that says a relationship is active must be periodically re-observed rather than living forever on an old link.