Serialize And Submit Task Variables Endpoint

Raw values pushed from a runtime — a kernel post-run hook, an explicit submit_variable call, or a script process.

Clients send the JSON-decoded value as-is; the server is the source of truth for type classification (see quadrillion.services.value_classifier). This keeps Python, R, and Julia qualia clients in lockstep regardless of how each language natively represents numeric literals.

A runtime identifies itself with notebook_id (kernels) or session_id (scripts, which have no notebook). At least one is required; when both arrive the notebook wins, as the more specific claim about where the value came from.