truapi-host: make the development product identity contract explicit

Part of #517.

Context

truapi-host dev derives localhost:<app-port> as the product id. That is correct only when the product uses its local-origin identity. A locally served version of a deployed .dot product must pass its production product id to the host and use the same value in the app.

The host derives the product account from the product id and refuses to sign for any other one, so a mismatch surfaces late, as a PermissionDenied on the first signature. Current documentation also contains contradictory claims about whether host and app configuration can drift.

Required behavior

• Documentation distinguishes local-origin products from deployed .dot products.
• Examples show --product-id dim2.dot and the matching product-side identity.
• Startup output shows the product identity being served.
• A product-id mismatch is reported at the earliest observable point, with an identity-specific diagnostic rather than a generic permission error.
• Documentation no longer claims that host and app configuration cannot drift.

Related: #515.

Start with the truapi-host dev entry point and the documentation work tracked in #517, then compare the local-origin and deployed .dot cases described here. The work is done when examples use a matching --product-id dim2.dot, startup output exposes the identity, mismatches produce an identity-specific early diagnostic, and contradictory drift claims are removed.