feat(credence_bond): validate governance approval invariants with reg…

[Derekwalter999]

Description

Ensures governance approval gating for parameter changes is correct and fully tested. Implements robust invariants for approver validation, proposal expiry, and category mapping.

Closes #1026

Type of Change

• feat — new functionality
• fix — bug fix
• docs — documentation only
• refactor — code restructuring with no behaviour change
• test — test additions or improvements
• ci — CI configuration changes
• chore — maintenance, dependencies, tooling

How Has This Been Tested?

• cargo test --workspace passes
• cargo fmt --all -- --check passes
• cargo clippy --workspace --all-targets --all-features -- -D warnings passes
• Coverage ≥ 95% for affected crates (cargo llvm-cov --package <crate> --fail-under-lines 95)
• Fuzz harness passes (cargo test -p credence_bond fuzz::test_bond_fuzz -- --nocapture)
• Error code wire-stability test passes (cargo test -p credence_errors error_codes_wire)
• Release build passes (cargo build --release)

Checklist

• Tests added/updated for new or changed functionality
• Docs updated (if public API, storage layout, error codes, or architecture changed)
• CHANGELOG.md updated (if contracts/** touched)
• Branch follows <type>/<short-description> naming convention
• Commit messages follow conventional commits

Additional Context

• Moved the GovernanceApproval struct and validate_governance_approval checks into governance_approval.rs to consolidate gating logic.
• Achieved >95% line coverage for the new governance invariants in test_governance_approval.rs.

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