Add precompute contract fixtures

[leoromanovsky]

Motivation

FFE consumers need one shared fixture suite that covers the precompute API response contract, not only SDK-style single-flag evaluation. The precompute response can be produced by multiple implementations, including edge evaluation and SDK-side precompute paths, and those implementations should converge on the same request and response bodies instead of carrying local copies.

Changes

This PR adds canonical precompute contract fixtures under precompute-cases/. Each case includes the JSON:API precompute request shape and the expected full precompute response. The fixture set covers assignment reasons, allocation metadata, created-at handling, value serialization, empty targeting keys, regex targeting, JSON values, and deprecated-but-present response fields.

The repository README now documents how downstream consumers should load precompute-cases/*.json: initialize an evaluator from ufc-config.json, call the precompute API with request, and compare the full response to expectedResponse.

Decisions

These fixtures live in precompute-cases/ rather than the older SDK-oriented precomputed/ shape because the contract here is the API response itself. SDKs can still layer typed-evaluation and event-emission assertions on top, but precompute producers should first prove that they emit the same JSON:API response for the same UFC config and request.

The temporal allocation cases use DEFAULT for active date-window-only allocations. startAt and endAt are scheduling metadata, not targeting rules, so they preclude STATIC without implying TARGETING_MATCH.

Validation

Fixture validation is performed downstream in dd-source by pinning a merged main commit from this repository and running the Rust evaluator/precompute contract tests against these cases. The JSON fixture in this PR also parses cleanly with python3 -m json.tool.