Docs: Android Play internal as the TestFlight analog

[erik-sutton95]

Maintainer task. Parent: #140. Product discussion: #139

Goal

Operators and agents can find the Play internal path the same way they find TestFlight.

Do

• docs/android-distribution.md — flow, version codes, secrets table, local bundle, kill switch (OpenZCine file, no Wear)
• Pointer from ANDROID.md (replace “not on Google Play yet” with internal vs waitlist when the track exists)
• docs/RELEASE.md — Play internal next to TestFlight; versionCode CI-stamped
• docs/ROADMAP.md — move “Play internal testing” from Next to Now when the first tester install lands
• Handbook apps/android.md + guides/setup.md — closed-beta waitlist vs internal testers; keep visitor copy honest
• Site waitlist copy only if the public story changes (don’t promise a store listing)
• AGENTS.md Read-when if a Play/signing pointer is missing (hygiene already covers keystores)

Out of scope

Implementing the workflow (sibling). Production Help listing.

Done when

A new session can ship an Android tester build from the docs without reading OpenZCine.

Verify

Handbook build. Links in ANDROID.md / RELEASE.md resolve.