Fix cross-change contract revalidation context

[piaro]

Problem and change

Contract Health could request revalidation of a clause created by another Change, while the Change-filtered Snapshot omitted the Contract needed to distribute its text and source digests.

Retain current Contracts across Changes in the Snapshot and keep action Context selection scoped to applicable or explicitly requested clauses. Also recognize explicit clause references when no applicability subjects overlap. Document that Contract change_id identifies origin and that existing Impact Assessments may need refreshing when the Contract index expands.

Validation

• Regression coverage verifies cross-Change clause text, scope, authority, evidence mode, source digests, and requirement mapping through Evidence registration and Result submission.
• Explicit verification covers unverified, stale, and failed clauses, including empty applicability scopes; existing selection tests exclude unrelated clauses.
• cargo fmt --check, cargo clippy --all-targets --locked -- -D warnings, cargo test --locked, and sh scripts/tests/test-rust.sh passed. HTTP server tests required execution outside the sandbox.
• The original reproduction currently requests a refreshed Impact Assessment before issuing revalidation requirements, so the original missing-clause set still needs checking after that refresh.