fix: AlpRd propagates left_parts validity; null float rows no longer decode as 0.0 (#234)

[dfa1]

What

AlpRdEncodingDecoder (fastlanes.alprd) dropped the validity mask on its left_parts child: null floating-point rows decoded as 0.0 instead of null — silent corruption. This is the same null-loss family as #210 (ALP), #225 (RunEnd), and #226/#232 (Sparse).

How

Following the #210 template, after decoding the left_parts child:

• if it is a MaskedArray, capture its .validity() and unwrap to .inner();
• build the lazy ALP-RD result over the unwrapped inner values (unchanged);
• if a mask was captured, re-wrap the result in new MaskedArray(decoded, validity).

An ALP-RD array's validity IS its left_parts child's validity, matching the Rust ValidityChild contract.

Tests

New AlpRdEncodingDecoderTest:

• null left_parts → null output rows (mask survives, valid rows decode via the dictionary);
• all-valid left_parts → plain DoubleArray, no MaskedArray wrapper (no regression).

All 1192 reader unit tests pass.

Conformance

Flips bi-euro2016 from gap:234 to ok in the Raincloud conformance matrix.

Closes #234.

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