Add Featureflip Java SDK to Feature Flags

[jhoward321]

Adds one entry to ### Feature Flags in README_SOURCE.md.

- [Featureflip Java SDK](https://github.com/canopy-labs/featureflip-java) - Java SDK for the Featureflip hosted platform, evaluating flags locally from a streamed rule set with an OpenFeature provider available.

Disclosure: I work on Featureflip, so this is self-promotion. Against the criteria:

• Java as a first-class target, not incidental. Featureflip is a multi-language platform, so this is the criterion worth scrutinising. The Java SDK is a dedicated library published to Maven Central as io.featureflip:featureflip-java, with its own CI and test suite, alongside a separate io.featureflip:featureflip-openfeature provider. It is a hand-written SDK, not a generated wrapper over a REST API.
• Distinctive from the other entries in the category. FF4J and Togglz are self-hosted libraries; the OpenFeature Java SDK is the vendor-neutral API itself. Against the two hosted-vendor SDKs, the description names what differs here: flags are evaluated locally in-process against a rule set streamed to the SDK, rather than by calling out per evaluation, and an OpenFeature provider ships alongside for anyone who wants to stay vendor-neutral.
• Licensing. Apache-2.0, so the SPDX chip should resolve automatically. No badge added by hand.
• Commercial, with clear pricing and a free tier. Pricing is public and flat at https://featureflip.io/pricing/, including a free tier.
• English documentation, at https://featureflip.io/docs/sdks/java/.
• Canonical GitHub repository used, alphabetical placement, description ends with a period, no trailing whitespace, one project per pull request, and README.md left alone since it is generated.

Happy to adjust the wording, or to withdraw it if you would rather the category stayed with the more established entries.

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Summary by cubic

Adds the Featureflip Java SDK to the Feature Flags category in README_SOURCE.md, giving Java developers a hosted option that evaluates flags locally.

• Flags are evaluated in-process from a rule set streamed to the SDK, with an OpenFeature provider (io.featureflip:featureflip-openfeature) for vendor-neutral use.
• Published to Maven Central, Apache-2.0 licensed, with public pricing and a free tier.
• The contributor works on Featureflip; treat this entry as self-promotion.

^{Written for commit dc30aba. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

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