test(opy): consolidate compatibility test metadata

[e54-bot]

Summary

• make each corpus fixture the single source/compiler differential test declaration
• remove duplicated expectation tables and generic evidence/provenance metadata
• reconcile compatibility docs and supersede ADR-0002 with the tests-first contract

Verification

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-targets --all-features
• python3 -m unittest discover -s tools/overpy/tests (28 passed)
• native differential corpus: 73 fixtures passed, including 3 declared known gaps
• independent ablation: removing tests.source from one fixture fails manifest loading

Oracle snapshot validation matched reference behavior and outputs. Two pre-existing snapshots differ only in transient pnpm install stdout ordering on this host; snapshots were not rewritten.

Fixes #337

[Teakowa]

1. tools/overpy/run_oracle.py:96 still requires an attribution object with kind/origin/license/redistributable for every fixture, and the synthetic manifests keep that boilerplate. #337 explicitly limits source attribution/licensing metadata to cases with a concrete third-party/redistribution/reproducibility need; this preserves the generic metadata schema under a new name. Make attribution optional and validate it only where the fixture actually needs it, then remove the WrightKit-authored synthetic attribution records.

2. The ADR cleanup AC is still incomplete. Accepted/current material still uses the generic concepts being removed: docs/adr/0003-first-party-provider-boundary.md:19 says diagnostics/provenance, docs/adr/0005-opy-canonical-program-lowering.md:25 refers to a public provenance contract, and docs/adr/README.md:54 says implementation evidence. Replace these with the concrete concern (source mapping/source identity/tests/current implementation reality as applicable). Historical wording inside superseded ADR-0002 can remain.

[Teakowa]

LGTM