Make DialCacheConfig modes discriminated and explicit

[lan17]

Priority

P2 — Medium — important hardening or API work after the safety foundations.

Problem

Contradictory or ignored constructor states are type-valid: Redis accepts both client and createClient; custom or disabled metrics can be combined with Prometheus prefix/registry options that are ignored; and constructing DialCache implicitly registers Prometheus collectors unless metrics: false is supplied.

Evidence:

• Redis options
• Only the neither-client case is rejected
• Metrics option matrix
• Implicit Prometheus construction

Open decisions

Confirm the metrics default and whether supplying both Redis client modes has any intended meaning.

Acceptance criteria

• Model Redis as exactly one of caller-supplied client or lazy factory.
• Model metrics as one of disabled, custom adapter, or Prometheus configuration.
• Reject ambiguous states in TypeScript and at runtime for plain JavaScript.
• Add positive and negative declaration tests to the packed-consumer suite.
• Document global registry side effects and the chosen default.