Gate slotConstraints behind PRO license entitlement

[nathanacurtis]

Problem

The processing.slotConstraints feature runs unconditionally based on the config flag alone — there is no license entitlement check. Consumers on the free tier receive slot constraint promotion (minItems, maxItems, anyOf promoted onto slot props) without a PRO license, inconsistent with how other premium processing features are gated.

Solution

slotConstraints processing should require a PRO-tier license, consistent with features like invalid combinations and slot content stamping. Free-tier output should leave constraint code-only props as-is rather than promoting them.

Acceptance criteria

• slotConstraints: true in config produces no slot constraint promotion when running without a valid PRO license
• slotConstraints: true with a valid PRO license promotes constraints onto slot props as expected
• Free-tier behavior matches other ungated processing features (config flag is respected, license gate is enforced)

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Implementation details are tracked internally.