## Why
Buzz server components need one typed feature-flag contract without
forcing the open-source build to depend on LaunchDarkly. A staging or
production relay must not be able to choose the wrong provider through
runtime configuration.
## What
- Adds typed boolean and signed `i64` flags with static, environment,
and compile-gated LaunchDarkly evaluators.
- Defines an executable relay artifact contract that compiles exactly
one provider.
- Rejects zero or multiple relay provider features at compile time.
- Runs the LaunchDarkly feature tests and the provider compile matrix in
the unit CI lane and fallback test script.
## How
`buzz-feature-flags` stays provider-neutral. The future relay
composition root will expose one build path per artifact feature:
`static-feature-flags`, `environment-feature-flags`, or
`launchdarkly-feature-flags`. Runtime configuration carries only
settings for the selected provider. It never carries a provider name,
fallback, or precedence rule.
The crate still has no relay consumer. A locked compile fixture proves
each singleton builds and every zero or multi-provider combination fails
until the relay adopts this contract.
This removes the proposed runtime `ProviderMode` and leaves one
provider, one construction path, and one evaluator per artifact.
## Risk
This PR still has no relay or app consumer, so it changes no
user-visible behavior. Risk is limited to the new feature-flag crate,
its compile contract, and its unit-test lane.
## Testing
An external Rust consumer at `30cf968bfe85e82980fda7e63f5bebe649ca939f`
exercised six environment-evaluator scenarios and 59 assertions. The
compile-time correction adds no live consumer to exercise manually.
## Bigger picture
When `buzz-relay` adopts this crate, its build manifests must map each
distribution to one provider feature. That consumer PR must verify live
Relay Proxy reachability, secret ingestion, lifecycle ownership, and
user-visible behavior.
Generated with Codex
---------
Signed-off-by: tornquist <tornquist@squareup.com>
Co-authored-by: Codex <noreply@openai.com>
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>
