Align Windows installer validation with the R2 release distribution

[Teakowa]

Goal

Make the Windows first-party installer and its distribution validation use WrightKit's R2-backed release surface consistently with install.sh.

Context

install.sh now resolves and downloads Wright releases from https://releases.wrightkit.dev, including latest/version and immutable versioned release paths. install.ps1 and its test fixture still model GitHub Releases/GitHub API URLs, so the Windows path no longer exercises the same supported distribution contract.

Scope

• Make install.ps1 use the WrightKit R2 release surface by default.
• Resolve latest through the R2 latest/version contract rather than the GitHub Releases API.
• Use the same R2 path shapes as the Unix installer for pinned and latest artifacts.
• Update the Windows installer test fixture to model the R2-backed distribution contract while remaining hermetic and retaining its existing failure-path coverage.

Non-goals

• Changing release artifact names, target support, or checksums.
• Adding a second release/distribution pipeline.
• Replacing hermetic failure-path fixtures with production-network-dependent tests.
• Changing package-manager distribution channels.

Acceptance criteria

• A default Windows install no longer depends on GitHub Release asset/API URLs.
• Pinned Windows installs resolve releases/<version>/... from the WrightKit release base URL.
• Latest Windows installs resolve latest/version and install from the verified latest/... artifact path.
• The Windows installer test validates the R2 path contract for pinned, latest, missing-version, checksum-failure, and native post-install smoke behavior.
• install.sh and install.ps1 share the same public release-distribution contract without introducing a new abstraction layer.

Dependencies / ownership

• Wright owns first-party installer behavior and distribution validation.
• Builds on the existing R2 publication contract already used by install.sh.