feat: project resource-bound receipt evidence

[tcconnally]

Summary

• Adds the resource/evidence contract for this repository.
• Preserves legacy compatibility and hash-only durable evidence where applicable.

Verification

• Local focused tests pass on the current branch.
• Full CI is the merge gate.

Related to #183

[tcconnally]

CI triage: the resource-constraint receipt tests pass locally (35 passed), and the failing matrix is currently reporting the repository's pre-existing full-suite failures in test_provider_usage_baseline_idempotency_and_report and test_dashboard_billboard_gate_and_tipjar (July-period fixture/dashboard expectations). The new receipt tests are not the failing tests. This PR is otherwise mergeable but remains blocked by the full engine matrix.