feat(proxy/store): price the scanned window from provider-counted usage

`caveman learn` could rank token sinks but never say what they cost. Add a
spend block that prices the window from the usage numbers the provider itself
counted, at rates from the dated shared/provider-catalog row, stamped with that
row's catalog version.

The honesty constraints are structural, not stylistic:

- Buckets are disjoint, so nothing is double-counted.
- A model with no catalog row is never priced at a sibling's rate. Its tokens
  are disclosed separately and the total is stated as a floor.
- Sinks are priced at the user's own measured effective input rate rather than
  list price, so a well-cached user's findings are not inflated roughly 10x.
- The figure is window-bounded and never projected into a month or a run rate,
  and it is never written as `verified` — that basis stays gateway-only.

Spend is what the window cost. It is not a savings claim; that needs an
attribution method, which lands separately.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>