Add release hygiene: CHANGELOG, mima, CI matrix, NOTICE, examples

[EtaCassiopeia]

Summary

Implements #128 (C1 — CHANGELOG), #129 (C2 — sbt-mima), #130 (C3 — CI matrix), #131 (C4 — NOTICE), and #132 (C5 — examples/) from the 1.0-readiness milestone.

Stacked. Base branch is feat/typed-errors-and-ofrep-validation (#139). This branch also merges in feat/init-timeout-and-sync-verify (#138) because C5's examples/ofrep-init-timeout uses the new initTimeout factory and OFREPProvider.make validated construction. Once #138 and #139 land in main, retarget to main.

C1 — CHANGELOG.md

Keep-a-Changelog 1.1.0 format. [Unreleased] is populated with the in-progress 1.0-readiness work (init hardening, typed errors, OFREP validation, optimizely module, WireMock failure suites, property + concurrency specs, mima, CI matrix, examples, NOTICE). Prior versions are summarised in a [0.8.0] — earlier block pointing at GitHub Releases for the full per-version detail. A "Maintaining this file" footer codifies the contract: every user-visible change must add an [Unreleased] entry in the same PR.

C2 — sbt-mima

• project/plugins.sbt: added com.typesafe % sbt-mima-plugin % 1.1.4.
• build.sbt: ThisBuild / mimaFailOnNoPrevious := false; mimaPreviousArtifacts := Set.empty in commonSettings. The first post-mima release tag will populate mimaPreviousArtifacts per module; until then mimaReportBinaryIssues is a no-op (verified locally).
• CI runs sbt +mimaReportBinaryIssues as a dedicated job so binary-compatibility regressions are caught at PR time once a baseline exists.

C3 — CI matrix

.github/workflows/ci.yml split into four jobs:

• format — scalafmtCheckAll once on JDK 21.
• build — matrix of JDK 17 + 21 × Scala 2.13.16 + 3.3.4 (4 cells, fail-fast: false). JDK 11 deliberately omitted: Optimizely's core-httpclient-impl requires JDK 17+, which is now the realistic floor.
• mima — runs +mimaReportBinaryIssues once.
• examples — compiles both example projects to keep README/docs snippets honest.

C4 — NOTICE

Apache 2.0 attributions at the repo root: OpenFeature Java SDK, OFREP contrib, Optimizely SDK (core-api + core-httpclient-impl), ZIO, ZIO Cache, Typesafe Config, plus WireMock under a test-only block since it isn't redistributed.

C5 — examples/

Two reference projects under examples/, both with publish / skip := true:

• examples/ofrep-init-timeout/ — production wiring with OFREPProvider.make + FeatureFlags.fromProviderAsync(_, evaluationTimeout). The CircuitBreaker pattern is documented (in the scaladoc preamble) but not wired in, because OfrepProvider extends FeatureProvider directly, not EventProvider, and CircuitBreakerProvider requires EventProvider. The example notes this and points to the Optimizely integration where the breaker does compose.
• examples/testkit-app/ — a trivial UserService gated by a flag, plus a spec that drives it through TestFeatureProvider.scopedLayer with setFlag / setStatus. Three test cases cover the off-by-default path, the flag-on path, and the provider-in-Error failure path.
• examples/README.md — covers running each example and the criteria for adding new ones.

Closes #128
Closes #129
Closes #130
Closes #131
Closes #132