docs: add Windows + invocation-path coverage to tests/MANUAL_TEST.md

[DanielPodolsky]

Problem

tests/MANUAL_TEST.md has no Windows section — and v2.6.0 pre-release testing proved why that matters twice:

1. PowerShell 5.1 encoding crash (powershell -File on the BOM-less .ps1) — invisible to all 100+ automated tests, found only by a human on real Windows
2. "Tested" is always "tested via a specific path": the documented irm <url> | iex install and the on-disk powershell -File install are different code paths with different encoding behavior. v2.5 was manually tested via irm | iex only, so the -File path had never been executed by anyone.

Proposal

Add a Release Verification section to tests/MANUAL_TEST.md codifying the v2.6.0 procedure:

• Windows (PowerShell 5.1 explicitly, not just PS7):
  • Install via irm | iex (documented path) AND via powershell -File (path printed by the scripts themselves)
  • Run bash tests/run.sh in Git Bash (note the WSL-bash trap: bash in PowerShell may resolve to System32's WSL launcher)
  • Dashboard: verify Ctrl+K label + palette, boot sequence, keyboard nav
• macOS: mirror checklist with ⌘K label
• Note which checks are platform-conditional vs shared

Origin: v2.6.0 release-process audit (PR #17).