Add len checker for exact length

[cinar]

Proposal

min-len/max-len exist but there's no single-tag way to require an exact length (phone numbers, fixed-width codes, postal codes). Add len:

• Tag syntax: checkers:"len:10"
• Applies to strings, slices, and maps, matching how min-len/max-len already handle those kinds
• Register a schema_maker.go mapping: len:N → minLength/maxLength both set to N (or minItems/maxItems, minProperties/maxProperties depending on kind, mirroring how min-len/max-len already branch)
• Add error code + message to all 23 locale files
• 100% test coverage; CHANGELOG.md entry

Follow the established checker file pattern (see min_len.go, max_len.go).