Allow creating clients for D14N

[api-Hypernova]

Allow enabling D14N.

[macroscopeapp]

Add environment-driven D14N client creation and require XMTP_API_URL when XMTP_D14N is "true" or "1" in helpers/versions.ts

Introduce helpers.isD14NEnabled, switch helpers.regressionClient to select d14nHost or apiUrl from env with validation, and default worker apiUrl from XMTP_API_URL; add D14N config guidance in .env.example.

📍Where to Start

Start with helpers.regressionClient and helpers.isD14NEnabled in helpers/versions.ts, then review worker URL selection in workers/manager.ts.

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Macroscope summarized fa2dba4.