Implement the section filter

[jzonthemtn]

identifiers.sections (filterSection, with startPattern and endPattern) has no filter and is not in _CUSTOM_SECTIONS, so a section filter in a policy is ignored.

Acceptance Criteria

• A section filter redacts text between startPattern and endPattern.
• sections is registered alongside identifiers, dictionaries, and pheyes.

[jzonthemtn]

Closes a gap against the PhiSQL specification. No RFC: $defs.filterSection and identifiers.sections predate v1.0, with SECTION a reserved keyword in spec/v1.0/catalog/keywords.yaml.