what

• create regex-assembly file for rule 921120 (HTTP Response Splitting detection)
• response header names decomposed into a named sub-assembly, composed with CR/LF prefix and colon suffix via sequence
• toolchain auto-optimizes content-type/content-length into content-(?:type|length)
• add standard "generated from regex-assembly" comment block

why

• maintainability: .ra file makes it easy to add/remove response header names
• consistency: aligns with the ongoing effort to move rules to regex-assembly format

refs

• Move regexes to regex-assembly format before LTS #4480