Price tier-resolved usage or record a structured unpriced reason #66

Problem

Model resolution can succeed while an event remains unpriced, without recording why pricing could not be applied. Tiered context rates, cache categories, or incomplete request-level usage can make a single exact price unavailable.

Impact

Consumers cannot tell whether a zero or missing cost means free usage, incomplete telemetry, an unsupported pricing rule, or an accounting defect.

Proposed remediation

• Add a typed unpriced_reason for every resolved-but-unpriced event.
• Preserve the selected tier, component rates, formula revision, and pricing-catalog revision used for each calculation.
• Use request-level usage when tier selection depends on request properties.
• When only aggregate usage is available, emit a documented bounded estimate instead of an unexplained missing value.
• Distinguish exact, estimated, unavailable, and excluded pricing states.

Acceptance criteria

• Synthetic fixtures cover requests below a tier boundary, above it, and aggregate usage spanning the boundary.
• A resolved event cannot be silently marked pricing-complete without an exact cost or typed reason.
• Exact and bounded-estimate calculations are deterministic for a fixed catalog revision.
• Reports expose pricing state without requiring raw provider payloads.

Privacy constraints

Tests and documentation must use invented models, rates, and usage values only.