docs: add contributor installation verification checklist

## Goal Add a contributor-facing installation verification checklist that complements `docs/INSTALLATION_VERIFICATION.md` without duplicating it. ## Scope Document a short, reproducible checklist for contributors to record after a local documentation or setup change. It should cover the root-module commands, the expected `open-stream-saver --help` result, optional FFmpeg verification for HLS/DASH remuxing, and the unpacked extension popup load. Keep the checklist suitable for Linux, macOS, and Windows. ## Acceptance criteria - Explain that the Go module is the repository root while the CLI package remains under `cli/`. - Use the canonical CLI package path; do not recommend `cli/vX.Y.Z` version suffixes. - State expected success signals, not just commands. - Keep FFmpeg optional and local. - Do not ask contributors to provide private URLs, cookies, credentials, tokens, protected media, DRM material, or browser-site permissions. - Link the checklist from the most appropriate existing contributor or installation document. ## Out of scope Do not add downloader capabilities, authentication handling, DRM support, access-control bypasses, or automated media tests.