Replace pricing draft with a thin pricing API

[JimCollinson]

Summary

• Serve one dated Inventory record through a bounded KV read and basic validation; no request-time collection, calculation or evidence replay.
• Retain valid historical pricing without age-based withdrawal.
• Share root and llms discovery descriptions while preserving existing supply formats, wallet accounting, caching, provider fallback and health behavior.
• Add ordinary no-deploy tests and the pinned Proposed ADR governance baseline.

This replaces the approach in the earlier draft #3, which is left untouched as a reference.

Hermes review

Review this together with the Inventory PR #3 and website PR #11.

Shared fresh-context handoff — fresh checkouts, repository setup, actual exercise, exact SHAs and command results. The handoff and Inventory repository require organisation access. Hermes review is pending; the local Fable review was not run and is explicitly replaced by this owner-selected route, not marked passed.

Evidence and boundaries

• Branch CI at 93bd5e6: 67 tests, pinned Wrangler dry-run and ADR checker passed. New PR checks are separate.
• Existing supply behavior is compared against the actual base Worker. Integrated adversarial and final Craft reviews passed; see the shared ledger.
• Production configuration, deployment workflow and legacy files are unchanged. No pricing binding or data was provisioned.
• Both ADRs remain Proposed. Keep draft pending Hermes and human decisions. No merge/deploy authorization: merging main triggers the existing deployment workflow, and the two public hostnames are the same Worker, not separate staging.