Add eq/ne literal value checkers

[cinar]

Proposal

eq-field/before-field/after-field etc. compare against a sibling struct field, but there's no checker for comparing a field against a literal constant. Add eq/ne:

• Tag syntax: checkers:"ne:admin", checkers:"eq:active"
• Simple equality/inequality against the string parameter
• Register schema_maker.go mappings: eq → const, ne → {"not": {"const": ...}}
• Add error codes + messages to all 23 locale files
• 100% test coverage; CHANGELOG.md entry

Follow the established checker file pattern (see email.go).