feat: FPS counter over the game through RTSS

[lippdev]

What

FPS counter drawn by RivaTuner (RTSS) in Console Mode's style, picked in Settings or cycled with A on a new "FPS counter" row of the session menu (Select + Y).

• Styles: Afterburner (default, changes nothing), Off (nothing on screen), Compact, Detailed (API, FPS, frame time) and Custom (items, one line or stacked, number color — set in Settings, applied live during a session).
• Written straight to RTSS shared memory (RTSSSharedMemoryV2, per the RTSS SDK) in our own OSD slot (ConsoleMode); needs RTSS, not rtss-cli.
• Except on "Afterburner", MSI Afterburner's OSD is hidden during the session: RTSS can't hide a slot and Afterburner's OSD hotkey turns the whole OSD off, so its slot text is emptied right after each rewrite (~1 s). It comes back by itself when ours is off or the session ends; Afterburner's settings aren't touched.
• Also bumps to 1.6.0-beta.7.

Verification

• dotnet test: 345 passed (new RtssOverlayTests); app builds on Windows.
• Slot write/release checked against a live RTSS with Afterburner running (Afterburner keeps slot 0, ours takes slot 1, release leaves slot 0 intact); measured Afterburner's rewrite period (~1 s).
• In-game behaviour tested by the owner with a portable build; checklist added to docs/TESTING.md.

🤖 Generated with Claude Code

[pullfrog]

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