feat(plugins): read the v3 plugin manifest from updates.tablepro.app

[datlechin]

Why

Release rewrite step 21. Registry plugins move to the release train: each v* tag writes plugins/v3.json on updates.tablepro.app through step 17's registry.py. The app still read the per-architecture plugins.json in TableProApp/plugins.

Change

• The registry client reads https://updates.tablepro.app/plugins/v3.json. URLCache (20 MB on disk) is the only cache, decoded at launch. One coalescing fetch(policy:) replaces the freshness window, the manifest file and the extra requests per install. The custom registry URL and the legacy key migrations go.
• A release is picked by channel (a -beta.N app also takes beta), minimumSystemVersion and PluginKit range, then the highest build. noCompatibleBinary becomes noCompatibleRelease.
• The Export dialog offers the formats the manifest lists in exportFormatIds instead of a hard-coded Parquet entry.

Verified

RegistryReleaseSelectionTests replaces RegistryBinarySelectionTests; the client, driver availability and catalog tests run on v3 fixtures. Deleted with their subjects: RegistryCategoryDecodingTests, RegistryClientURLTests, the registry migration case in StandardDomainIsolationTests.

Not in this PR

• Merges in the freeze window with steps 20-23. Debug builds from main show an empty plugin catalog until step 24 publishes the manifest.
• registry.py does not write exportFormatIds yet, so no format is offered for install until it does.
• Docs: docs(development): describe the plugin train and the v3 manifest docs#33.