feat(panorama-viewer): make Registry the sole release source

[superche]

Summary

• establish packages/plugins/panorama-viewer as the only source for the product's HTML, CSS, JavaScript, WebGL renderer, manifest, localized title, toolbar commands, tests, package metadata, and release ZIP
• bump the Registry release to panorama-viewer@0.2.1 and Registry sequence 29
• document the host/plugin ownership boundary and the legacy built-in migration contract
• keep the package static, offline, sandboxed, and limited to the generic host capabilities it declares

Why

Panorama Viewer previously existed both here and inside Convax Desktop. That dual source made releases ambiguous and caused the Desktop built-in identity to collide with the same-id Registry package. This PR makes convax-plugins the sole source and publishing boundary.

Host compatibility

The companion host PR removes the built-in catalog reservation and migrates a verified legacy installation by dropping only its built-in provenance marker. This release is intentionally 0.2.1, newer than the final built-in 0.2.0, so the normal Registry update transaction replaces the preserved package.

Companion host PR: https://github.com/microvoid/convax/pull/42

Validation

• bun run check — workspace builds/typechecks/tests, validate, companion builds, 67 tooling tests, all-package pack, Registry and Showcase index passed
• bun test tooling/panorama-viewer.test.js tooling/registry.test.js — 10 passed, 0 failed, 105 expectations
• bun run validate — 19 packages at Registry sequence 29
• bun run pack -- --kind plugin --id panorama-viewer — 72,588-byte ZIP, SHA-256 4e876c9753708a10058eb42eabbd70694121158c997d2542047aa8cdbf979e81
• bun run build:index — Registry sequence 29 with 19 packages; Showcase sequence 29 with 12 packages
• real Convax Electron acceptance from the generated Registry index and exact packed ZIP: installed panorama-viewer@0.2.1 with trustedBuiltin: false; initialized WebGL with a 1024×512 local panorama; captured the viewport to .convax/assets/全景视口截图.png; persisted one image node and one plugin → image edge at revision 3
• GitHub Actions Validate / check — passed

Merge order

1. Merge https://github.com/microvoid/convax/pull/42 first.
2. Merge this PR.
3. Tag/release plugin-panorama-viewer-v0.2.1.
4. Publish Registry sequence 29 only after release coverage verifies the immutable ZIP.