Epic: Clustering expansion — HDBSCAN, OPTICS #453

Tracking issue for density-based clustering work. Do not start code before an RFC is agreed here.

Member issues

• Embed HDBSCAN #291 — HDBSCAN
• OPTICS algorithm #52 — OPTICS

Why grouped

Both algorithms build on DBSCAN-style reachability and need neighbor/range queries. Design the shared reachability + query plumbing once; note this is not newcomer work (the good first issue label was removed from #291 for that reason).

Current state (main @ a955334)

Clustering lives in src/cluster/: K-Means, DBSCAN, Agglomerative. The neighbors/ module (cover tree, KD tree, brute force) is a candidate building block — check its fit before adding new index code.

Constraints (repo conventions)

• Target small/average datasets with a limited memory footprint.
• Pure Rust, no macros; zero-copy iteration via view traits; numbers/ traits only.
• Doctests + mod tests {}; parity checks against reference outputs where licences allow.

RFC requirements

Post a short RFC comment before any PR: public API, cluster-selection strategy (e.g. EOM vs leaf), memory plan, no_std impact.

Acceptance criteria

• RFC agreed by a maintainer
• Members implemented or explicitly descoped
• CHANGELOG updated