fix: use getAllInterfaceFields in findInterfaceByDiscriminant for inherited discriminant fields

[cs01]

Summary

• findInterfaceByDiscriminant was passing iface.fields (own fields only) to checkInterfaceForDiscriminant
• This caused discriminant union narrowing to miss interfaces where the discriminant field is inherited from a parent interface (e.g., interface Child extends Parent where Parent has type: 'literal')
• Fixed to use this.getAllInterfaceFields(iface) which includes all inherited fields
• Adds test fixture interface-extends-discriminant.ts verifying discriminant field access on interfaces with inheritance

Test plan

• 432 tests pass
• Full 3-stage self-hosting passes

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