[10/11] Add len exact-length checker

[cinar]

Replaces #233, which GitHub auto-closed (rather than retargeted) when its base branch (p1-09-contains-family) was deleted after #232 merged. Same content, rebased onto current main.

Problem

min-len/max-len exist but there was no single-tag way to require an exact length — phone numbers, fixed-width codes, postal codes.

Fix

Add len, mirroring min_len.go exactly:

• The generic Len[T any] signature is usable standalone with any Len()-able type, and uses the same v.Len() (byte length, not rune count) semantics already used by min-len/max-len — kept consistent with its siblings rather than introducing a rune-counting discrepancy between the three.
• Registered in maker.go.
• Mapped in schema_maker.go to matching Min/Max pairs (MinLength=MaxLength, MinItems=MaxItems, or MinProperties=MaxProperties, depending on kind), since an exact length is just min=max.
• NOT_LEN message added to all 23 locale files plus en-US.
• Documented in the README's checker table.

Testing

• len_test.go: standalone generic usage, string/slice struct-tag valid/invalid, bad-param panic, non-Len()-able-type panic.
• TestJSONSchemaLenExactLength covering string/slice/map, plus a bad-param panic test.
• Full suite passes, 100% coverage maintained.
• go vet, gosec, revive clean.

Fixes #207

🤖 Generated with Claude Code

https://claude.ai/code/session_01FLdVmP5daHiknrTW4Geh2i

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (ca5b02f) to head (cc57f04).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #236   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           63        64    +1     
  Lines         1163      1196   +33     
=========================================
+ Hits          1163      1196   +33     

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