Enable Microsoft Store distribution (lift the MSIX publish pause)

Summary

MSIX release publishing is disabled in CI. Once packaging tooling lands (#1328), enabling
distribution is a separate, gated decision. This issue tracks that work.

Current state

• .github/workflows/ci.yml:811 - build-msix is if: false # MSIX distribution is paused; ship Inno setup and portable ZIP artifacts only. This predates #1328 and was not changed by it.
• docs/RELEASING.md:115-118 requires packaged camera/microphone consent validation plus release coverage before distribution resumes.
• docs/RELEASING.md:23-26 is a checklist step that asserts those paused strings exist, so it must be edited in the same change that lifts the pause.
• Tagged releases currently produce Inno installers and portable ZIPs only. The latest release carries no MSIX asset.

What lifting the pause requires

1. Packaged camera, microphone, and location consent validation under package identity (proof pool windows-winui-interactive). Not captured in #1328.
2. Native ARM64 install and launch proof (proof pool windows-11-arm64). The ARM64 artifact builds but has never been run.
3. Clean install and version upgrade proof on signed release artifacts (proof pool windows-clean-installer-upgrade).
4. Re-enable build-msix and update the RELEASING.md checklist assertion together, so validation does not fail on the stale paused strings.
5. Partner Center work outside this repository: listing, age rating, published privacy policy URL, and the written runFullTrust justification.

Not in scope

Packaging tooling itself, which #1328 delivers, and Inno-to-Store coexistence and migration, tracked in #1374.

Decision needed

Whether to lift the pause at all, and if so whether Store becomes the primary channel or runs alongside the existing .exe channel indefinitely.

Start with .github/workflows/ci.yml:811 and docs/RELEASING.md:23-26,115-118 to understand the paused MSIX checks and release requirements. Review the listed proof pools for consent, ARM64, clean installation, and upgrade validation, then resolve the Store-channel decision. Done requires the repository changes, release evidence, and the specified Partner Center work.