Implement Spark bootstrap for IVF clustering and MDT posting generation

Parent: #19094
Related: #18676

## Summary
Implement Spark bootstrap for IVF clustering and MDT posting generation.

## Scope
- centroid training
- vector assignment to centroids
- posting generation
- quantizer and manifest rows
- generation write path

## Acceptance Criteria
- index bootstrap produces a complete generation
- bootstrap output is persisted to MDT
- bootstrap logic is separate from read-path logic

Start by reading parent issue #19094 and related issue #18676, since the payload names no files, tests, or code entry points. Trace the requested centroid training, assignment, posting, quantizer, manifest, and generation write-path components, and consider the work complete when a full index generation is produced and persisted to MDT with bootstrap logic separate from read-path logic.

Written by the indexing model from the issue text.