Refactor JanusQL parsing around an AST

[argahsuknesib]

Summary

• replace regex-based JanusQL clause parsing with an explicit AST-backed parser
• keep the existing ParsedJanusQuery output shape so the rest of Janus continues to work
• add focused AST parser tests for register parsing, multiline window clauses, ON LOG sliding windows, and nested WHERE window bodies

Validation

• cargo fmt
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-features