feat(compatibility): add pinned OverPy feature-contract inventory

[e54-bot]

Summary

• add a pinned OverPy feature-contract inventory with 257 registry leaves and 25 distinct parser/compiler/tooling branches
• validate immutable reference pins, registry drift, required branches, production paths, and leaf-level evidence
• expose inventory gaps in conformance reports and document the audit workflow

Tests

• python3 -m unittest discover -s compatibility/tests
• pinned-source inventory validation against OverPy commit 889d9749d1def17f146548cbddb94ea1ab015847
• python3 -m py_compile ..., JSON validation, and git diff --check

The full compiler conformance runner was not executed because the local target/debug/opy-cli and target/debug/opy-compat binaries were unavailable.

Fixes #192

[Teakowa]

1. compatibility/feature-contracts.json: the claimed exhaustive pinned inventory omits src/data/opy/stringEntities.ts#opyStringEntities. The pinned compiler consumes that registry for user-visible \&name; string escapes in src/utils/strings.ts, so #192 AC requiring every user-visible feature family is not met. Add this contract surface with provenance/status/coverage/evidence, and ensure an omitted whole registry/family is detectable.

2. compatibility/conformance.py::validate_inventory: compiler-branch completeness is only checked by comparing branches with requiredBranches, both supplied by the same inventory. Removing a branch from both makes the audit accept the omission; --upstream-root only validates key sets for already-declared registries. This does not satisfy #192 AC requiring reporting to detect an upstream compiler branch absent from the inventory. Anchor branch completeness to independent pinned-upstream audit evidence and add a regression that removes the branch from both inventory lists.

3. compatibility/tests/test_conformance.py::test_feature_contract_inventory_is_leaf_explicit: the new test hard-codes 257, 25, and 282 as stable assertions even though #192 explicitly does not make feature counts the contract and the updated docs say the inventory is not a fixed feature-count assertion. Replace these counts with invariants tied to the independently audited inventory so a legitimate pin update does not require changing an implementation-detail count alongside the inventory.