feat: gate primitives behind feature flags

[tisonkun]

Summary

• Make every public primitive module an opt-in Cargo feature and keep default = [].
• Encode feature dependencies between composite primitives and make hashbrown optional for the once and singleflight paths.
• Gate shared internal modules, tests, docs, and benchmarks so zero-feature, individual-feature, and all-feature builds remain clean.
• Add an xtask/CI feature matrix modeled after DataSketches Rust and document feature selection for users.

Breaking changes

• Downstream users must explicitly enable every primitive they use.
• Raise the MSRV from Rust 1.85.0 to 1.86.0, matching the cargo_metadata version used by the feature-matrix tooling.

Validation

• cargo x check
• cargo x test
• cargo +1.86.0 x test
• cargo x lint
• cargo x build --locked

Closes #102.