Add DirectBountyEvidenceChecklist

[devyeyostellar]

Fixes #686

Adds DirectBountyEvidenceChecklist struct and its validation logic to domain crate to satisfy the direct-bounty evidence checklist requirement.

[NSPG13]

The PR is in the manual-security-review lane because it adds a large executable validator surface. What passed: it attempts to solve the evidence-checklist problem. What blocks main: the scope is substantially larger than the issue and overlaps #688, #739, and #745. Action: reduce the PR to the smallest schema, fixtures, and validator needed for #686; run cargo run -p cli -- docs-contract-check and focused tests; explain any migration. Done when the smaller diff has deterministic fixtures and no payment authority. Thanks for helping the community grow, and sorry for the review friction.