feat: productionise L1C custom-AC composite as S2L1CGeeSource (v0.6.0)

[MarcYin]

Add a Sentinel-2 L1C custom atmospheric-correction composite as a first-class package source, reusing the existing scout/select/windowed-fetch/compositor machinery (the same path as sources/s2_gee.py for L2A).

• atmosphere.py: AtmoSidecar + SceneAtmosphere — load the per-scene 6S sidecar, select_low_aod clean-day gate, (tile,date) lookup so a GEE system:index matches a sidecar keyed by earth-search STAC id, and vectorized 6S TOA->BOA correction (pure numpy, mirrors the Rust atcorr core).
• sources/s2_l1c_gee.py: S2L1CGeeSource — lists L1C + Cloud Score+ scenes, restricts to the lowest-AOD fraction, scouts coarse Cloud Score+, fetches raw TOA via getPixels (no compute graph, ~2.5x faster than computePixels for raw assets), 6S-corrects each chunk, and emits Observation(data=BOA, quality).
• sources/s2.py: S2_L1C constants + aod_cloud_score_to_quality — gate on Cloud Score+, then rank clear pixels by lowest MAIAC AOD (most reliable correction) with cs as a deterministic tie-break (cs saturates near 1.0 on clear pixels).
• cli.py/types.py: wire --gee-product s2_l1c --atmosphere-sidecar --low-aod-frac.
• scripts/run_l1c_composite.sh: sidecar generation (3 stages) + the CLI build.
• tests: test_atmosphere.py, test_s2_l1c_gee.py, plus aod-quality coverage.

Validated live against GEE: the CLI composite matches the earlier prototype within ~2-3 DN (Cairo blue 1338 vs 1336, red 2244 vs 2241). 97 tests pass.