release: distribute rc1 beta to NDA users and open feedback channel

[justinjoy]

Context

#685 lists "NDA beta distributed to >=3 external users; feedback channel running" as an rc1 freeze exit condition. Existing #699/#714 cover migration-guide validation, but they do not cover beta program operations: recipient roster, artifact/instruction distribution, feedback channel setup, or triage ownership.

Scope

Track the operational rc1 beta distribution and feedback loop. Keep private user-identifying details outside public issue text; record privacy-safe evidence only.

Acceptance

• At least 3 external NDA beta recipients are confirmed in maintainer-private evidence.
• The rc1 beta artifact, tag, or install instructions distributed to those users are recorded in privacy-safe form.
• Feedback channel is created and announced to recipients.
• Feedback triage owner and expected response loop are documented.
• This issue cross-links migration-guide feedback tracked in Blocker B12: Migration guide externally validated #699 without replacing it.

Refs: #685, #699, #714.

[justinjoy]

Current dependency note after #900/#714 cleanup:

#896 remains the canonical rc1 blocker for beta distribution operations. Public closure evidence should stay privacy-safe and should include:

• at least three external NDA recipients confirmed in maintainer-private evidence;
• the exact rc1 artifact/tag/commit/install instructions distributed, preferably after the final B17: 1.0 branch cut + branch-protection rules #746 1.0 cutover;
• feedback channel name/type without exposing private participant details;
• triage owner and expected response loop;
• cross-link to Blocker B12: Migration guide externally validated #699 for migration-guide validation results.

Do not close this issue on documentation readiness alone; it needs distribution/feedback evidence.