refactor(contracts): move runtime-fact exports into their own manifest #1418

Gap Claim

implementations/python/packages/raes_contracts/contracts/_exports.py is 499 lines. Files under implementations/python/packages are capped at 500 lines (tools/policy/adr_policy.yaml oversized_source_files), and tools/policy/oversized_allowlist.yaml is empty and may only shrink. Publishing another contract family through the raes_contracts.contracts facade adds one import and one spread entry to this manifest. The inject-trigger and occurrence contracts for #1366 would therefore take the file to 501 lines and fail the repository policy.

Earlier families avoided the cap by keeping their names in their own manifest modules: _backend_operation_exports.py (#1388), _mixed_composition_exports.py (#1336) and _execution_policy_exports.py (#1413).

Existing Surface Audit

• _exports.py lines 420–437 hold one contiguous family: RUNTIME_FACT_BINDING_PLANE_V1_SCHEMA_VERSION and the 17 RuntimeFact* names defined in contracts/runtime_facts.py. No other family member sits elsewhere in the manifest.
• contracts/__init__.py imports these symbols explicitly from .runtime_facts and ._version_exports. It does not need to change.
• implementations/python/tests/test_contracts_facade_exports.py requires contracts.__all__ == PUBLIC_EXPORTS, unique names, and a resolvable attribute for every name. test_conformance_facade_parity.py checks the facade's exported set.
• The API reference documents individual modules with automodule, not the facade's __all__.

Alternatives

1. Add the Publish inject-trigger and occurrence contracts #1366 names to an existing family manifest such as _backend_operation_exports.py: rejected, because it mixes unrelated families.
2. Add _exports.py to the oversized allowlist: rejected, because the allowlist may only shrink.
3. Move the contiguous runtime-fact family into _runtime_fact_exports.py and spread it at the same position: selected.

Chosen Architecture

Add _runtime_fact_exports.py defining RUNTIME_FACT_EXPORTS with the same 18 names in the same order, following _mixed_composition_exports.py. _exports.py imports it and spreads it where the names were. PUBLIC_EXPORTS keeps identical membership and order, so no public name, schema, fixture or runtime behavior changes. _exports.py drops to 483 lines.

Verification Plan

• Compare PUBLIC_EXPORTS before and after the change for identical order and membership.
• Run test_contracts_facade_exports.py and test_conformance_facade_parity.py.
• Run repository fast feedback against origin/dev.