feat(ws_network): add Ws_SetBroker for runtime broker override

[nicolas-rabault]

Adds a writable 128-byte buffer behind the existing s_url pointer and exposes ws_hal_set_broker / Ws_SetBroker to override the compile-time WS_NETWORK_BROKER_ADDR default. Must be called before Ws_Init; no effect afterwards (Ws_Init captures the URL during mg_ws_connect).

Additive change: existing callers that rely on the compile-time default are unaffected.

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