truapi-host: document and optionally streamline custom bridge configuration

Part of #517.

Context

The one-line 127.0.0.1:9955/bootstrap.js tag is a good default for a single host. A second host necessarily uses another URL, so product tooling must inject or select the matching bridge endpoint.

Required behavior

• Documentation limits the "no environment variables" claim to the default single-host setup.
• Multi-host examples show separate sessions, host ports, app ports, product identities, and matching bridge tags.
• Changing --port clearly tells the developer that the product tag must use the same port.

Optional follow-up

• Define and export non-secret child-process configuration such as the bridge URL and product identity from truapi-host dev.
• Let Vite, Next.js, or similar tooling consume those values, so the CLI becomes the configuration source.
• Treat variable names and compatibility guarantees as a separate public API decision.

Start by locating the truapi-host documentation and the existing 127.0.0.1:9955/bootstrap.js example, then inspect the truapi-host dev entry point and its --port guidance. Update the docs to cover the single-host limitation, complete multi-host values, and matching ports; treat child-process configuration and public variable names as a separate optional decision.