Use Bifrost declaration navigation

[DavidBakerEffendi]

UsageBench still treated Bifrost declaration navigation as unsupported after Bifrost added the distinct get_declarations_by_location MCP tool. This updates the adapter to exercise the current declaration contract and score its exact locations.

Key Changes:

• Route explicit declaration lookups to get_declarations_by_location while preserving the existing definition and profile-default paths.
• Parse declaration candidates and no_declaration responses, advertise native declaration lookup capability, and retain machine-readable unknown-tool failures.
• Replace the obsolete unsupported-path test with native dispatch and missing-tool regression coverage.

Touch Points:

• src/runners/bifrost.rs

Validation:

• cargo fmt -- --check
• cargo test — 93 library tests, 2 CLI tests, and doctests passed using a consistent Rust 1.97.1 toolchain.
• cargo run -- validate benchmarks/cases — 35 case documents validated.
• Full run against Bifrost 9620eb8f: 132 passed, 13 failed, 3 expected failures, 2 unsupported, 0 position-unverified, and 0 runner errors. Eighteen declaration cases moved from unsupported to passed.