docs(opy): remove stale compatibility contract residue

[e54-bot]

Summary

• remove retired compatibility-baseline and support-matrix documentation and references
• make the canonical docs/language-support.md contract and linked inventories the only current support authority
• describe conformance, differential, oracle, and compiler reports as evidence contracts while preserving their executable machinery
• reconcile the cleanup with the current architecture documentation on main

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-targets --all-features — 370 passed
• python3 -m unittest discover -s tools/overpy/tests — 34 passed
• python3 tools/overpy/conformance.py --binary target/debug/opy-cli --semantic-binary target/debug/opy-compat --report target/opy-rs-conformance-report.json — exit 0
• live retired-reference audit and pre-cleanup ablation check

Fixes #201

[Teakowa]

LGTM