Add rule: prefer_single_param_onChange

[kj6dev]

Summary

Detect .onChange(of:) modifiers using the two-parameter closure when only the new value is used.

Problem

// Bad - oldValue is unused
.onChange(of: someValue) { oldValue, newValue in
    handleChange(newValue)
}

// Also bad - underscore shows intent but still uses wrong API
.onChange(of: someValue) { _, newValue in
    handleChange(newValue)
}

Preferred

// Good - single parameter closure
.onChange(of: someValue) { newValue in
    handleChange(newValue)
}

// Good - direct method reference
.onChange(of: someValue, perform: handleChange)

Detection Logic

1. Find .onChange(of:_:) calls (the two-param version)
2. Check if first closure parameter is:
   • Unused (no references in body)
   • Named _ (explicit discard)
3. If either, trigger warning

Origin

Discovered via vector DB analysis of AGENT directives in edit logs:

• "must not use the two parameter onchange when you're only using the new data"
• "I meant .onChange(of: ..., perform: ...)"

Implementation Notes

• SwiftSyntax rule in CustomRules package
• Could offer auto-fix to single-param version
• Related to closure indirection pattern (separate rule candidate)