spc_011-C-02: prompt token measurement — the three-way split the spec’s §6.1 invariant
(“measure provider-visible input, never underestimate”) requires between what a request is
estimated to cost before it is sent (PromptMeasurement, this module), what the provider
actually reports back (ProviderUsage, Node/Python Host layer — not this crate). This module
only defines a preflight fact. The MeasurePrompt wire tag is reserved, but A-00R removed its
scheduler producer until request fingerprinting and durable replay semantics are defined.