Codebase cleanup: CI, docs, lints, dedup, tests

[gvonness-apolitical]

Summary

Comprehensive codebase cleanup implementing the full cleanup plan (Phases 0-4):

• CI: Feature matrix testing (default, --all-features, --no-default-features), remove orphaned codecov.yml, clean up commented-out jobs
• Documentation: Populate empty CHANGELOG.md, add # Example sections to 8 convenience functions, add 5 crate-level doc sections (tanh-sinh, Cauchy PV, oscillatory, weighted, sparse grids)
• Pedantic lints: Resolve all 613 pedantic clippy warnings — underscore separators on 323 numeric literals, 103 #[must_use] annotations, 45 doc_markdown fixes, 41 errors/panics doc sections, ~90 cast/misc fixes
• Code deduplication: impl_rule_accessors! macro for 9 rule types (~160 lines removed), QmcSequence trait unifying Sobol/Halton methods (~100 lines removed), shared compute_clenshaw_curtis replacing duplicate in sparse_grid (~35 lines removed)
• Testing gaps: 15 new tests — weighted integration over non-default domains, high-dimensional Sobol sequences (20/30/40-dim), adaptive integration with reversed bounds and break points

Final state: 194 unit tests + 51 doc tests, 0 clippy warnings (default and pedantic), clean formatting, docs build without warnings.

Test plan

• cargo test (default features) — 194 + 51 pass
• cargo test --all-features — 194 + 51 pass
• cargo test --no-default-features — 187 + 50 pass
• cargo clippy --all-features -- -D warnings — 0 warnings
• cargo clippy --all-features -- -W clippy::pedantic — 0 warnings
• cargo fmt --all -- --check — clean
• cargo doc --no-deps — builds without warnings