feat(sinsemilla): accept a pre-decoded first word

[ValarDragon]

Summary

• add UncheckedFixedLengthHashDomain::hash_with_first_word
• accept an already-decoded first Sinsemilla word and decode only the remaining bit suffix
• cover full 52-word parity and suffix zero-padding

Motivation

Orchard Merkle hashing already has its 10-bit level word as an integer, while its two 255-bit child encodings remain a bit string. The existing weighted API forces downstream users either to re-encode the level as bits or to duplicate the complete fixed-word decoder. This hybrid entry point keeps the layer word intact and leaves generic Sinsemilla word chunking inside zakura-sinsemilla, without adding Orchard-specific semantics to the crate.

Performance boundary

A release-mode comparison of 10,000 weighted evaluations was flat when the suffix bits were already available (103.4 ms through hash_words, 102.6 ms through hash_with_first_word). Replacing Orchard’s specialized byte decoder with a field-bit iterator was 2.1% slower in the existing orchard-merkle-combine benchmark (8.15 µs versus 7.98 µs), so this PR deliberately adds the missing API without changing Orchard’s optimized scalar or batch implementation.

Tests

• cargo fmt --all -- --check
• cargo test -p zakura-sinsemilla
• cargo check -p zakura-sinsemilla --no-default-features