Preserve quant compressor API for explicit PQ generation

[xwj-ox]

• Does this PR have a descriptive title that could go in our release notes?
• Does this PR add any new dependencies? No.
• Does this PR modify any existing APIs? No public API changes; this preserves the existing constructor-based quant generation API.
• Is the change to the API backwards compatible?
• Should this result in any changes to our documentation, either updating existing docs or adding new ones? No.

Reference Issues/PRs

Follow-up to #1299.

What does this implement/fix? Briefly explain your changes.

Preserves the existing quant compressor and quant data generator API while keeping disk-search PQ pivot generation explicit inside DiskIndexBuilder. This removes the lazy generate(&self) lifecycle and OnceLock path introduced in #1314-style changes, avoiding the concurrent initialization race without adding a mutex or public API.

Any other comments?

Verification:

• cargo fmt --all --check
• cargo test -p diskann-disk
• cargo clippy -p diskann-disk --all-targets -- -D warnings