rpn: revert resolveMetric and helpers to single return values

resolveMetric was changed from returning *Metric to (*Metric, error) but
the error path only fires when Cool is missing from the registry, which
is an impossible runtime state (previously a panic). This added ~90 lines
of boilerplate error handling across multiple files.

Revert resolveMetric, coolMetric, baseMetric, resultMetricForMul,
resultMetricForDiv, compatibleMetric, and convertFromBase to single
return values that panic on impossible states. convertToBase retains
its error return for reachable error cases (type assertion, Float64).

0 files changed, 0 insertions, 0 deletions