feat(core): add between and lengthBetween validation functions

[nil957]

Summary

Adds two new convenience validation functions for common range checking scenarios.

New Functions

between

Check if a numeric value is within a range (inclusive).

{
  "type": "between",
  "args": { "min": 1, "max": 100 },
  "message": "Value must be between 1 and 100"
}

lengthBetween

Check if a string length is within a range (inclusive).

{
  "type": "lengthBetween",
  "args": { "min": 3, "max": 50 },
  "message": "Must be between 3 and 50 characters"
}

Helper Functions

Also adds corresponding check helpers:

check.between(1, 100, 'Out of range');
check.lengthBetween(3, 50, 'Invalid length');

Motivation

Currently, checking both upper and lower bounds requires two separate checks:

{
  "checks": [
    { "type": "min", "args": { "min": 1 }, "message": "Too low" },
    { "type": "max", "args": { "max": 100 }, "message": "Too high" }
  ]
}

With between, this becomes a single check with a single error message:

{
  "checks": [
    { "type": "between", "args": { "min": 1, "max": 100 }, "message": "Must be 1-100" }
  ]
}

Checklist

• Tests added for both functions
• Tests added for check helpers
• No breaking changes

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