Fix installer compatibility with Windows PowerShell 5.1

[merefield]

Why

The Windows installer fails before downloading anything in Windows PowerShell environments where RuntimeInformation.OSArchitecture is unavailable. Requiring PowerShell 7 would add an unnecessary installation prerequisite.

Changes

• Keep runtime OS architecture detection when available; fall back to native Windows architecture environment variables when the property or type is unavailable.
• Prefer native architecture over shell architecture and reject unsupported or undetermined architectures rather than guessing.
• Add architecture mapping tests for AMD64, ARM64, emulated shells, missing runtime information and unsupported values.
• Run the end-to-end installer tests under both Windows PowerShell 5.1 and PowerShell 7 in CI and release workflows.
• Allow expected child-process failures to reach exit-code assertions under Windows PowerShell stderr handling.
• Document both supported PowerShell editions and how to check the installed version.

Validation

• git diff --check passes.
• PowerShell is not installed in the local Linux environment; Windows execution is delegated to the new CI checks. No claim of local Windows validation.
• Existing installer tests retain checksum validation, installed binary version checks, invalid version rejection and directory-target protection.

No changes to the Go application, terminal interface or release version.

[Copilot]

Copilot review overview

🟢 Approval recommended

No unresolved blocking issues were identified.

Review effort: Lite
Findings: None

What changed in this PR

Updates the Windows installer for compatibility with Windows PowerShell 5.1 and PowerShell 7.

Changes:

• Adds robust architecture detection and validation.
• Expands installer testing across both PowerShell editions.
• Documents supported PowerShell versions.

File	Description
test/​install-release.ps1	Adds architecture and failure-handling tests.
README.md	Documents PowerShell compatibility and version checks.
install-release.ps1	Adds architecture resolution and fallback logic.
.github/​workflows/​release.yml	Extends release validation to both PowerShell editions.
.github/​workflows/​ci.yml	Tests the installer under both PowerShell editions.

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