release: v0.2.0 (#118)

Bump version 0.1.1 → 0.2.0 (pyproject.toml, cng_datasets/__init__.py,
docs/conf.py) and write the 0.2.0 changelog entry (CHANGELOG.md +
docs/changelog.md, promoting the prior Unreleased section).

First tagged release. 0.2.0 collects the correctness/feature/infra work
accumulated since the untagged 0.1.1 — notably mass-conserving raster→H3
aggregation (#84, breaking reducer-default change), BLOB→GEOMETRY and
row-unique _cng_fid fixes across both convert paths (#61, #43), hex-write
UBIGINT assertion + _cng_fid ordering (#102, #103), and multi-cluster /
YAML-profile configuration (#57, #58).

Co-authored-by: Claude Opus 4.8 (1M context) <[email protected]>