feat: SelfValidator for custom and cross-field validation

[mlwelles]

What this adds

SelfValidator, an opt-in seam that lets a type drive its own validation —
building on the existing StructValidator support rather than replacing it.

type SelfValidator interface {
    ValidateWith(ctx context.Context, v StructValidator) error
}

When a value passed to Insert/Upsert/Update implements SelfValidator,
the client calls ValidateWith instead of handing it straight to the configured
StructValidator. Plain structs are unaffected — they still flow through
StructCtx exactly as before.

The problem it solves

Struct tags validate one field at a time. Plenty of real validation lives
outside what a tag can express:

• Cross-field rules — one field constrained by another (End >= Start,
  password == confirm).
• Conditional rules — field B required only when field A has some value.
• Computed / setter-derived values — state that is not a plain tagged field.
• Business rules — anything that needs real logic.

SelfValidator is the seam for those. ValidateWith receives the configured
StructValidator, so an implementation can run ordinary tag-based validation
first and layer custom logic on top:

func (e *Event) ValidateWith(ctx context.Context, v mg.StructValidator) error {
    if v != nil {
        if err := v.StructCtx(ctx, e); err != nil { // tag-based checks first
            return err
        }
    }
    if e.End < e.Start { // then the cross-field rule
        return fmt.Errorf("End (%d) must be >= Start (%d)", e.End, e.Start)
    }
    return nil
}

validateStruct routes each element through a validateOne helper that detects
SelfValidator (on the value or its address) and otherwise falls back to
StructCtx — unchanged behavior for ordinary structs.

Tests

• TestValidateRoutesToSelfValidator / TestValidateFallsBackToStructCtx —
  routing and the unchanged fallback.
• TestValidateSelfValidatorInSlice — slice elements route correctly, and the
  configured StructValidator is not also invoked (strengthened this round
  to assert the no-double-call, matching the scalar test).
• TestSelfValidatorCustomCrossFieldRule — a concrete cross-field example.

The automated review (cubic) found no issues in the implementation.

Documentation

A runnable ExampleSelfValidator showing a cross-field rule that layers on the
configured StructValidator.

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}