Document native Roaring bitmaps for Redis 8.12

[aviggiano]

Summary

• document native Roaring-compressed bitmaps introduced in Redis Open Source 8.12
• explain conversion rules, string-command compatibility, memory tradeoffs, and BITOP behavior
• document persistence and replication requirements
• add the native bitmap return value to the TYPE command reference

Why

The native Roaring bitmap feature in redis/redis#15331 is targeted for Redis Open Source 8.12. It adds an opt-in representation with an important compatibility boundary: bitmap commands work transparently, while generic string commands do not operate on native bitmap values. Users need that behavior and the rollout requirements documented before enabling the feature.

User impact

Users can evaluate whether Roaring compression fits their workload, enable it safely, understand when existing strings convert, and upgrade replicas and persistence consumers before rollout.

Validation

• git diff --check
• Hugo 0.143.1 rendered the site successfully
• inspected the rendered example, warning, headings, anchor, and internal links

[CLAassistant]

All committers have signed the CLA.

[dwdougherty]

Hi @aviggiano. Thank you for this PR. Which Redis Open Source release will contain this new feature? I know it's not in 8.10, because it's just about closed. Is it for 8.12?

Your PR target needs to be a release-specific feature branch, not main. Once you answer the above, I will create the appropriate branch and then you can edit your PR.

Also, this repo does not allow slashes (/) in branch names.

[aviggiano]

Thanks, it's still unclear which release this will be targeted

I'll sync with @sundb and will update the PR

[aviggiano]

Yes, this is for 8.12. I’ve retargeted the PR to feature-oss-8.12 and updated the copy. The existing head branch predates the slash restriction; renaming it would close this PR, so I kept it to update the existing PR.

[dwdougherty]

Thank you, @aviggiano. We actually fixed the branch naming issue just recently, so you should be good. Let me know when this PR is ready for review (it's still a draft).