Publish a kernel-less win32-arm64 VSIX

[samclark2015]

Stacked on #523.

Problem

Positron's bootstrap cannot reach the universal VSIX. getPlatformDownloads() in build/lib/extensions.ts unconditionally appends ?targetPlatform=<platform>-<arch> to the gallery asset URL, and the gallery matches that value exactly — in both directions.

Measured against the live registry (https://p3m.dev/openvsx/latest/vscode/asset/...):

request	result
posit.air-vscode 0.28.0 ?targetPlatform=win32-arm64	200
posit.air-vscode 0.28.0 ?targetPlatform=alpine-x64 (never published)	403
posit.air-vscode 0.28.0, no targetPlatform	403
ggsql.ggsql 0.4.1 (universal only) ?targetPlatform=win32-arm64	403
ggsql.ggsql 0.4.1 (universal only) ?targetPlatform=win32-x64	403
ggsql.ggsql 0.4.1, no targetPlatform	200

So omitting win32-arm64 does not degrade to the universal build — it 403s. fetchUrl (build/lib/fetch.ts) accepts only 2xx, retries 10×, then throws, which fails Positron's own Windows arm64 build. A WoA user never sees an error, because that build never ships the bootstrapExtensions entry.

There is also no way to scope around it: extension.platforms filters on process.platform (win32/darwin/linux) only and cannot exclude an architecture, so backlogging win32-arm64 means dropping Windows bootstrap entirely.

Change

Publish win32-arm64 as a target that carries no kernel. It answers 200, the bootstrapExtensions entry can land, and on that platform the extension falls back to a host-installed kernel — the behaviour every platform had before bundling. When a Windows arm64 runner exists, the kernel drops into the same target with no change on the Positron side.

The build-vsix matrix now carries an explicit kernel flag per entry instead of testing for universal by name, since two targets now ship without one.

Verification

Packaged locally from this branch:

$ vsce package --target win32-arm64
DONE  Packaged: ggsql-win32-arm64.vsix (15 files, 105.97 KB)

$ unzip -p ggsql-win32-arm64.vsix extension.vsixmanifest | grep -o 'TargetPlatform="[^"]*"'
TargetPlatform="win32-arm64"

$ unzip -l ggsql-win32-arm64.vsix | grep 'extension/bundled/'
(no output — carries no kernel)

The workflow's contents check was exercised in all four combinations — kernel-less/kernel: false passes, kernel-less/kernel: true errors, kernel-bearing/kernel: true passes, kernel-bearing/kernel: false errors.

Not run: the workflow itself, which only executes on a v* tag.

Out of scope

The old-glibc Linux gap is handled in the PR stacked on top of this one. The Install ggsql notification added there also covers a Windows arm64 machine with no kernel installed.

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