Implement RaBitQ encoder contract with multibit-aware scoring support

Parent: #19094
Related: #18676

## Summary
Implement the RaBitQ encoder and scorer contract with multibit-aware support.

## Scope
- 1-bit and multibit encoder contract
- encoding-version-based dispatch
- metric-aware scorer behavior
- validation and compatibility checks

## Acceptance Criteria
- multibit-aware encoding contract is defined
- scorer dispatch is explicit
- metric compatibility is enforced or rejected clearly

Start by reading the parent issue #19094 and related issue #18676 to understand the existing RaBitQ encoder and scorer contracts. Define done as a multibit-aware encoding contract, explicit encoding-version scorer dispatch, and clear metric compatibility validation, as specified by the acceptance criteria.

Written by the indexing model from the issue text.