fix(release): tolerate npm metadata propagation

[benlesh]

What changed

• wait for npm package metadata to expose a successfully published version before enforcing the exact SHA-512 comparison
• revalidate registry reads online and treat an unpropagated ETARGET like the observed new-package 404
• add regression tests for both propagation responses
• update the README now that RxJS 9 beta.0 is available under next
• record the four published integrities, verified commit, qualification run, npm channel state, publishing-access hardening, tag, and GitHub Release
• mark P6.10 complete and leave the completed project queue with no NEXT item

Why

npm accepted each new scoped package before its ordinary package metadata replaced a cached pre-publication 404. The release command therefore reported an integrity failure even though the exact version endpoint already held the correct immutable bytes. Immediate retries then received npm's expected duplicate-version 403.

Post-publication verification now waits up to ten minutes for that eventually consistent read path while preserving the exact integrity requirement. Unexpected registry errors and byte mismatches still stop the release.

Release evidence

• 9.0.0-beta.0 is public for all four packages
• all four SHA-512 values match the rehearsed tarballs
• every package's next tag resolves to 9.0.0-beta.0
• rxjs@latest remains 7.8.2
• all four packages require interactive 2FA and disallow automation-token publication
• tag 9.0.0-beta.0 targets verified commit 11d931645c03cc4b67eb70ee3a1122a53f8e0bc9
• GitHub prerelease: https://github.com/ReactiveX/rxjs/releases/tag/9.0.0-beta.0

Verification

• pnpm run release:check — 39 checks passed
• pnpm run test:workflows — passed
• focused beta release tests — 8 passed
• Prettier checks — passed
• diff hygiene — passed
• live npm exact versions, integrities, next, and latest — verified