refactor(widget): remove multi-instance mount check

[petar-omni]

Remove the runtime document claim check that threw an error when mounting more than one Widget Instance in the same browser document.

Hosts can now mount multiple widget instances concurrently without runtime errors. Update ADR 0001, ARCHITECTURE.md, and CONTEXT.md to clarify that multiple instances remain unsupported due to shared document-global state (CSS custom properties, wallet discovery, and portals).

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