v0.40.0

Feat

• dataloader: support variant-windows and flank tokens in mode='double_buffered'
• dataloader: pass _FlatVariantWindows through the double-buffered consumer reshape
• dataloader: shm kind=2 carries _FlatVariants.flank_tokens
• dataloader: shm kind=4 serialization for _FlatVariantWindows
• dataloader: replay VarWindowOpt and flank config in the producer schema
• dataloader: support variant-windows and flank tokens in mode='buffered'
• dataloader: instance-axis slicing for flat window and flank types
• dataloader: byte accounting for variant-windows and variants flank_tokens

Fix

• dataloader: propagate unphased_union to producer, Svar2 info-dtype guard + start accounting in variant-windows
• dataloader: AF-filter-safe dummy_variant byte accounting + direct bound test
• dataloader: store token_alphabet on Haps, drop lossy LUT inversion (restores double_buffered parity for colliding unknown_token)
• dataloader: replace contiguity heuristic with round-trip verification in _token_alphabet_from_lut
• dataloader: key producer-schema Config A/B on sequence_type, not stale window_opt
• flat-variants: accept str/bytes/NucleotideAlphabet for with_settings(token_alphabet=...)