Add native full-build v2 delta evidence

[weikengchen]

Summary

• add native v2 delta root-bundle payload and predecessor-free full_build evidence
• produce and bind non-empty Direct ORAM delta inputs before evidence generation
• reject roots-only delta builds that attempt to claim v2 full-build evidence
• extend the deterministic regtest fixture with a derived second snapshot and non-empty delta checks

Dependency

The snapshot full-build v2 foundation was merged in #8. This PR is now based directly on main and contains only the delta extension.

Validation

• scripts/local-regtest-e2e.sh
• cargo test -p pir-attested-builder (31 passed)
• bash -n scripts/build-delta-database.sh scripts/local-regtest-e2e.sh
• cargo check -p pir-attested-builder
• Rust formatting check
• git diff --check

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