feat(distribution): add Windows PowerShell installer

[e54-bot]

Summary

• add a first-party Windows x86_64 install.ps1 for latest or pinned releases
• download and verify the canonical release ZIP and published SHA-256 checksum
• install and directly smoke-check both wright.exe and wright-lsp.exe
• add a local HTTP controlled-artifact functional test and Windows CI wiring
• document the installer alongside the Unix installer

Verification

• python3 scripts/verify-dist.py
• shellcheck install.sh
• git diff --check
• YAML parsed successfully with Ruby YAML.load_file
• Windows-only PowerShell functional test is wired for windows-latest but was not runnable on this macOS host

Fixes #254

[Teakowa]

Major — install.ps1 installs by default into %LOCALAPPDATA%\Programs\Wright\bin, but it neither adds that directory to PATH nor tells the user how to make wright available by name. A clean user following the documented one-liner therefore gets a successful install but cannot invoke wright normally afterward. This misses the first-party installer/PATH-handling contract inherited from #108/#254. Add explicit PATH handling or an actionable post-install PATH instruction (at minimum matching install.sh).

[Teakowa]

LGTM