[6/11] Add gt/lt strict numeric comparison checkers

[cinar]

Problem

gte/lte cover inclusive numeric bounds, but there's no strict-inequality equivalent — a common gap for users coming from validators that offer both.

Fix

Add gt/lt, mirroring gte.go/lte.go exactly, including the CanInt/CanUint/CanFloat handling from #193 and the non-numeric-field-panics contract:

• gt.go/lt.go registered in maker.go under "gt"/"lt".
• Added ExclusiveMinimum/ExclusiveMaximum to the Schema struct (nothing existing covered them) and mapped gt/lt to them in schema_maker.go, matching JSON Schema's exclusiveMinimum/exclusiveMaximum keywords.
• NOT_GT/NOT_LT messages added to all 23 locale files plus en-US.

Bonus find, not fixed here: while writing these messages I noticed NOT_LTE's text is byte-identical to NOT_GTE's across every single locale file — a pre-existing copy-paste bug (lte's error currently says "cannot be less than X" when it should say "cannot be greater than X"). Left untouched since it's unrelated to this PR's scope; worth a small dedicated fix as a follow-up. Wrote correct, non-copy-pasted text for the new NOT_GT/NOT_LT messages so this PR doesn't propagate the same mistake.

Testing

• gt_test.go/lt_test.go: value/boundary/error cases including an explicit "equal to bound is an error" test (the actual difference from gte/lte), int/uint/float, non-string... non-numeric-field panic, bad-param panic.
• TestJSONSchemaGtLtExclusiveBounds plus bad-param panic tests for the new schema makers.
• Full suite passes, 100% coverage maintained.
• go vet, gosec, revive clean.

Fixes #203

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Chain note: PR 6 of 11.

🤖 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 (3511512) to head (b89e3b1).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #229   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           54        56    +2     
  Lines          983      1051   +68     
=========================================
+ Hits           983      1051   +68     

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