docs(pricing): specify pricing v1 contract

[hubab1]

Summary

• define bounded public App Store pricing semantics before schema or runtime work
• keep exact local prices primary and constrain ECB conversion to dated approximate comparisons
• specify incomplete visible-purchase coverage, provenance, caching, partial failures, UI, and read-only MCP acceptance

Review gate

This is a contract-only draft. It intentionally requires maintainer approval before the pricing schema, providers, UI, or MCP implementation proceeds. No runtime behavior or persisted schema changes in this PR.

Validation

• markdownlint-cli2: 0 issues
• reference, code-fence, and whitespace checks passed
• Apple, ECB, and source-history links verified
• independent contract review findings resolved

Source history

The feature direction was informed by WIP commit akshaynexus@5de98ea. This specification is an independent redesign; the link records provenance and does not imply co-authorship.