feat(u4): add little-endian bit adapter

[brenorb]

Summary

• add a mirrored 61-item staggered lookup table that emits each u4 nibble least-significant-bit first
• preserve checked range validation, stack scheduling, and the existing 1..=234 batch bound
• record the 32-nibble boundary: 924-byte script, 64-byte representative witness, 189-item peak, and 735 static non-push opcodes

Research result

The little-endian adapter matches the existing checked big-endian converter in script size, stack peak, and opcode count. It removes the need for a caller-side four-bit reversal when a ScriptNum or scalar consumer needs least-significant-bit-first nibble output. Witness nibbles remain hostile: checked mode enforces numeric range 0..=15, while canonical encoding and protocol binding remain caller obligations. Deployment remains unclassified; local evidence uses the repository tapscript executor with the strict stack limit.

Tests

• cargo test --locked arithmetic::u4::bits::tests -- --nocapture --skip batch_size_guard_matches_the_strict_stack_peak
• cargo test --locked arithmetic::u4::bits::tests::batch_size_guard_matches_the_strict_stack_peak -- --exact --nocapture
• python3 tools/kb.py validate
• UPDATE_PRIMITIVE_METRICS=1 cargo test --locked --test primitive_metrics (6 passed, 1 ignored)
• cargo test --locked --test primitive_metrics (6 passed, 1 ignored)
• full repository test suite intentionally not run