Mem-Client: as const Arrays Need Companion Union Type

Exported `as const` arrays require companion union type exports.

Rationale:
- Companion union types preserve type safety and prevent duplicated literal unions.

Scope:
- PR review policy for `domains/frontend/mem-client/src/**/constants.ts`.

Trigger:
- A PR adds or changes an exported `as const` array.

Approval checks:
1) Each exported `as const` array has an exported union type companion using `(typeof ARRAY_NAME)[number]` (or equivalent derived-literal type expression).

Evidence:
- Diff shows array export and companion type export together.

Decision:
- Return NOT APPROVED when trigger is met and the companion union type export is missing.