README: Windows-first install instructions, PowerShell examples, tutorial, and audit notes

[MLyte]

Motivation

• Improve onboarding for Windows users by prioritizing PowerShell-based, step-by-step installation instructions.
• Provide clear fallbacks for entry-point availability and network-restricted pip installs to reduce setup friction.
• Surface a short repository audit snapshot so readers know what checks were run and the current status of runtime dependencies.

Description

• Rewrote the Installation section as a Windows-first guide with virtualenv creation using py -3.11 -m venv .venv and PowerShell activation commands.
• Added explicit instructions to verify FFmpeg on PATH, an editable install step python -m pip install -e ., guidance for network/proxy failures, and a note that PySide6 is in dependencies.
• Added a short tutorial with numbered steps and PowerShell examples for optimaster CLI usage, module-mode fallbacks, GUI launch, and presets/config usage.
• Appended an "Audit "install now" (snapshot: 2026-04-22)" section summarizing checks performed in the repo snapshot and current limitations around ffmpeg and GUI imports.

Testing

• Running pytest -q in a fresh environment failed with ModuleNotFoundError: optimaster when the package was not installed.
• Running PYTHONPATH=src pytest -q passed with 4 passed tests.

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Codex Task