feat: SelfValidator for custom and cross-field validation

[mlwelles]

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

What

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

Why

Struct tags handle field-at-a-time checks, but plenty of validation lives outside what tags can express. SelfValidator is the seam for that:

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

ValidateWith receives the configured StructValidator, so an implementation can still run ordinary tag-based validation and layer custom logic on top.

Tests

TestValidateRoutesToSelfValidator, TestValidateFallsBackToStructCtx, TestValidateSelfValidatorInSlice, and TestSelfValidatorCustomCrossFieldRule (a concrete cross-field example) cover the routing, the unchanged fallback, and the custom-rule utility.

[cubic-dev-ai]

No issues found across 2 files

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