Port gRPC duration helper contracts

Tracks copied Bun corpus position 3978 from #66: js/third_party/grpc-js/test-duration.test.ts.

Acceptance criteria:

• expose the grpc-js duration build submodule
• implement duration message conversion, millisecond conversion, type guards, parsing, and canonical formatting
• preserve nanosecond precision and return null for invalid duration strings
• lower the typed expectation table, pass all four upstream checks, and pin the file permanently

Parent: #66

Completed in 29d587aeca319cfc3d192ea2fbe90c3682b28216 and pushed to main.

• Added the complete grpc-js duration build submodule: duration-message conversion, millisecond conversion, structural guards, string parsing, and canonical formatting.
• Fractional seconds preserve nanosecond precision, integral forms normalize correctly, and invalid strings return null.
• Added narrow TypeScript lowering for the upstream typed table and a permanent regression pin.
• Focused result: 4 passed, 0 todo, 0 failed, 0 unsupported.
• gRPC diagnostics, SQL contracts, and the complete permanent third-party suite pass.