[5/11] Add oneof checker

[cinar]

Problem

Without oneof, every enum-shaped field needs a hand-written RegisterMaker call — the single most commonly cited DX gap across every one of the six source reports behind this issue list.

Fix

Add oneof, following the established checker file pattern:

• IsOneOf is generic (IsOneOf[T comparable]) so it's usable standalone for any comparable type, not just strings.
• Syntax correction from the filed issue: the tag is oneof:admin,user,guest (comma-separated), not oneof:admin user guest (space-separated) as originally proposed. Checker tag fields are already split on whitespace before an individual checker's own params are parsed (maker.go's makeChecks does strings.Fields(config) over the whole tag first), so a space-separated value list literally can't be parsed by the existing tag DSL — it would be seen as multiple unrelated checker names. Followed the precedent already set by credit-card:visa,mastercard instead.
• The error message includes the allowed values via a generic newOneOfError helper (same pattern as the existing newGteError/newEqFieldError), so a failure is actionable instead of a bare code: "Value must be one of admin, user, guest.".
• schema_maker.go maps oneof to JSON Schema's Enum — a field that existed on Schema but nothing previously populated.
• NOT_ONE_OF message with an {{ .allowed }} placeholder added to all 23 locale files plus en-US.
• Documented in the README's checker table, with the comma-vs-space syntax called out.

Testing

• oneof_test.go: valid/invalid, generic non-string usage (int), error message content, empty-config panic, non-string-field panic, struct-tag valid/invalid.
• TestJSONSchemaOneOfEnum for the Enum mapping.
• Full suite passes, 100% coverage maintained.
• go vet, gosec, revive clean.

Fixes #202

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Chain note: PR 5 of 11 — branched locally on top of the now-merged uuid PR, then rebased onto current main so the diff here is only this PR's own changes.

🤖 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 (a87659f) to head (a4bf334).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #228   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           53        54    +1     
  Lines          958       983   +25     
=========================================
+ Hits           958       983   +25     

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