[cary-rowen]

Installing or upgrading Weasel has been observed to change the Windows display language or restore a previously removed Microsoft Pinyin input method. This PR changes how setup updates input method registration and the current user’s language list.

Microsoft documents that Windows dynamically adjusts display language and default input method settings based on the user language list, and recommends the International PowerShell cmdlets for updating these settings on a running system.

Changes:

• Replace the installation and uninstallation calls to InstallLayoutOrTip with a PowerShell helper that reads the existing list through Get-WinUserLanguageList, modifies Weasel entries, and writes it back through Set-WinUserLanguageList. Skip writing the list when no change is needed.
• Preserve TSF profiles during upgrades while updating and registering the components, avoiding removal and re-addition of the input method.
• Migrate the current user’s legacy IMM entries to TSF, retaining the earliest Weasel position and updating a default input method override that references the legacy entry. Keep shared IMM registration and binaries during upgrades so other accounts can still use them; remove them during uninstallation.
• If the selected Chinese language entry is absent, leave the language list unchanged and prompt the user to add it in Windows Settings during interactive setup.

Add helper unit tests and installer regression tests covering input method ordering, duplicate entries, repeated execution, legacy migration, preservation of shared resources during upgrades, and uninstallation cleanup. Installer tests also check the language list, display language override, and default input method override.

Closes #1296.
Closes #1849.

[cary-rowen]

cc @fxliang would love to get your thoughts on this when you have a moment.

[fxliang]

LGTM