Document encoder setup and report mpv startup failures

[ekisu]

Summary

Document that encoding launches a second mpv process and requires the player's inherited PATH to include the executable. Add Windows and macOS/Linux checks, distinguish shell PATH from desktop launch environments, and explain mpv's own codec availability.

Fixes #163 (requested installation-instruction revision).
Related support reports: #135, #188, #202, #77.
Tracked in #217.

Verification

• Checked the documented requirement against the "mpv" child command in src/encode.moon and both subprocess paths in src/util.moon.
• Checked authenticated issue comments documenting Apple Silicon PATH and mpv-vs-ffmpeg codec linkage.
• command -v mpv, mpv --ovc=help, and mpv --oac=help work on the local Linux installation.
• Markdown and whitespace reviewed. Documentation-only change; Windows/macOS setup steps are based on the platform reports, not newly executed here.

Latest rebase verification

Rebased onto master 72d9d11 after #227 merged. Normal build and the complete pinned-Nix suite pass, with two expected skips (audio fixture and unavailable optional libaom-av1 encoder).

Review feedback addressed — 2026-09-12

Added a linked step-by-step Windows encoder setup guide and an actionable startup error for a missing/unlaunchable mpv executable. A preflight runs before normal, progress, detached and two-pass launch paths. Real mpv is started by absolute path with an empty PATH to test the failure without mocks. Full pinned-Nix suite: 28 tests, 26 passed and 2 expected skips. GitHub build/test checks pass.
Rebased onto master after #220, #221, #223 and #224 merged.

[ekisu]

Could we also add a better error message when encoding fails due to mpv not being present?

[ekisu]

Approved.