feat(bestpixel): end-to-end L1C custom-AC composite (v0.2.0)

[MarcYin]

Add bestpixel.build_l1c_composite — a Sentinel-2 L1C custom atmospheric- correction monthly composite — to the published bestpixel package, with the 6S correction running in Rust and the GEE fetch in Python.

• Rust: new correct_toa(toa, xap, xbp, xcp) PyO3 function wired to the crate's atcorr 6S core; applies rho_boa = y/(1+xcpy), y = xaprho_toa-xbp per pixel, NaN nodata passthrough, negatives clamp to 0.
• python/bestpixel/l1c.py: build_l1c_composite — MAIAC clean-day gate, scout-first windowed getPixels of L1C TOA + Cloud Score+, correct_toa (Rust), best-pixel compose (lowest-AOD wins, cs tie-break), optional int16 GeoTIFF.
• python/bestpixel/_gee.py: lazy GEE fetch helper (getPixels patch, coarse scout, asset resolution from the sidecar's system:index).
• python/bestpixel/atmosphere.py: 6S sidecar loader (AtmoSidecar/SceneAtmosphere).
• init exports build_l1c_composite; init.pyi stub + README section.
• pyproject: 0.1.2 -> 0.2.0, numpy core dep, new [gee] extra (earthengine-api, pyproj, rasterio); Cargo.toml bumped to match.
• tests/test_l1c.py: correct_toa round-trip + the pipeline with a fake ee.

Sidecar built by scripts/{build_atmo_sidecar,custom_ac_phase2_sixs, atmo_sidecar_merge}.py; scripts/run_l1c_composite.sh runs it then the composite. Validated live against GEE: blue 1338 / red 2243 DN (matches the prototype). cargo test 11/11, bestpixel pytest 4/4, repo ruff clean.