fix: preserve requested DBN metadata version and align v3 output

[vbmithr]

Problem

Metadata.Write routes every non-v1 version to writeV2, which hardcodes the DBN\x02 prefix. Asking for v3 therefore silently produces v2 metadata, including when the following records use v3 layouts (notably statistics). Unsupported versions are silently downgraded too.

Change

• Preserve the requested version for v1, v2 and v3; reject unsupported versions before writing.
• Add the trailing metadata padding required for v3's eight-byte alignment, and include it in the encoded metadata length.
• Preserve existing v1/v2 layouts and document that record layouts must match the metadata version.
• Add tests for all three supported versions, all eight v3 alignment residues, the first record boundary, and invalid versions.

Validation

• GOWORK=off go test . ./live passed, including the existing Ginkgo suite.
• Application interoperability tests passed with Databento's dbn CLI 0.71.0 for trades, MBO, MBP1, MBP10, OHLCV and statistics. The statistics fixture uses a quantity above 32 bits to exercise the v3 layout.
• V3 version bytes, declared metadata lengths and record decoding were checked independently of dbn-go's scanner.

The alignment follows Databento's encoder.