Prefill: from the query parameter to the plan

What happens between the prefill query parameter and the HTML that is served. The contract — the two entry points, the error tables, what is base and what is temporary — is in prefill.md.

The walk

From the query parameter to the plan: json.loads() runs, the root is verified to be an object, decode() prepares the transport, and a temporary partial Signature (containing only the parameters present) builds them with build(), producing exact Python values. Those values go into page_of(), which generates the plan and the HTML. The Python entry point starts directly at the last step.