fix(s1-rtc): store nodata as NaN, not 0, so titiler masks it transparent

[lhoupert]

Why

S1 RTC previews render the out-of-swath region as opaque black while the S2 reference renders it transparent. The cause is the data, not the metadata: #201 already gave vv/vh the same dtype / fill_value=NaN / _FillValue / grid_mapping encoding as S2, but those are inert because no NaN is ever written — s1tiling stores 0.0 out of swath, and titiler treats 0 as valid data and paints it black.

	nodata in data	renders
S2 b04	NaN	transparent ✅
S1 vv (rc1, _FillValue)	0.0, NaN 0%	opaque black ❌

This is the data half; #201 was the metadata half.

What

Mask nodata → NaN at the writer (src/eopf_geozarr/conversion/s1_ingest.py):

• vv/vh — np.where(border_mask == 0, NaN, …) before overview generation. border_mask is the authoritative valid-data mask (0 = no-data). _downsample_2d already uses np.nanmean for floats, so NaN propagates to every overview level.
• float conditions (gamma_area/lia) — masked read off the GeoTIFF's declared nodata (border_mask is N/A for static conditions); a no-op when no nodata is declared.

No change to the create-store paths — only the two read sites are touched (12 src lines).

⚠️ Only newly re-ingested cubes get NaN. Existing cubes are remediated separately.

Tests (TDD, RED→GREEN)

tests/test_s1_rtc_ingest.py — all 4 fail on 082913a, pass after the fix:

• test_nodata_masked_to_nan (new) — NaN ⟺ border_mask == 0 at native and the r20m overview; valid pixels stay finite.
• test_fill_value_masking_roundtrip (rewritten) — masked region now comes from border_mask, not a pre-seeded NaN; round-trips via use_zarr_fill_value_as_mask.
• test_conditions_nodata_masked_to_nan (new) — declared-nodata → NaN.
• test_preserves_data_integrity (updated) — valid region preserved exactly, nodata region NaN.

Plan trio (test_s1_rtc_ingest + test_array_attrs + test_integration_sentinel1) = 85 passed; whole tests/ green (exit 0, optional-dep skips only).

Stacks on #201; targets #180.

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