feat: add configurable IVF and PQ training sample limits

[jerry-024]

Summary

Allow callers to configure IVF and PQ training sample limits independently through the shared option-map API. Both options default to the existing limit of 256 points per centroid.

Changes

Option	Applies to
ivf.train.max-points-per-centroid	Coarse K-means training for IVF-FLAT, IVF-SQ, IVF-RQ, and IVF-PQ, including hierarchical clustering stages
pq.train.max-points-per-centroid	PQ training for IVF-PQ and DiskANN, including PQ training inside OPQ

• Pass the configured limits through the trainer to the corresponding clustering stages.
• Reject non-positive or malformed values, sample-count overflow, and options unsupported by the selected index type.
• Document the options in the API and per-index parameter tables, fix Rust configuration examples, and add migration guidance.

Testing

• cargo test -p paimon-vindex-core training_max_points_per_centroid --lib — 3 passed; covers defaults, validation, configured training across all five index types, and OPQ.
• cargo test -p paimon-vindex-core config_ --lib — 14 passed.
• cargo test -p paimon-vindex-core --test storage_format_fixtures — 2 passed; fixture generator ignored.
• cargo clippy --all-targets --workspace -- -D warnings
• cargo fmt --all -- --check
• Extracted all 9 Rust configuration examples from docs/*.html into a temporary Cargo example and compiled them with cargo check --offline -p paimon-vindex-core --example docs_training_config_check.

Notes

• Rust source compatibility: Direct VectorIndexConfig enum literals must provide the new training-limit fields. Existing constructors and train methods retain their defaults.
• Existing trainer reservoirs, OPQ's input cap, and DiskANN's memory budget still constrain the available training data. Increasing these options does not raise those limits.
• These options affect training only and do not change the index file format.
• Native option keys have no fields.<field-name>. prefix. Paimon integrations need a separate option-filter change to forward these keys.