chore(overlay): ship a version pill only, with F8 to hide it

[Frostn1]

What

FrostMod's in-game overlay is now just a version pill, with F8 to hide it.

Before, the overlay was an F8 action menu plus four modal panels (bike model swap, track manager, track switcher, direct connect) and a radar/rider-outline HUD, all mirrored across two render paths. Bike model swaps are the MXB App's job — the in-game panel was a second way to do the same thing.

The pill

Shows	When
FrostMod v0.11.0 - attached	idle
/ Reloading mods... 60% + progress bar	while a reload runs
a status line	briefly, e.g. the MXB App asking you to re-select a bike

F8 hides and shows it, and is now the only key FrostMod reads anywhere — no digits, arrows, Enter/Esc or PageUp/PageDown — so it can't shadow a game binding or race another HUD plugin for a keystroke. Hidden means hidden: a reload in progress doesn't force it back.

How

A single compile-time switch, FROSTMOD_UI, at the top of src/frostmod.cpp — 0 ships, 1 restores the previous build verbatim. Every menu, panel, HUD renderer and keyboard block sits behind it. The diff is almost entirely #if markers; no feature code was deleted.

Both render paths — the GL wglSwapBuffers fallback and the sanctioned PiBoSo Draw() callback — are gated together, so they can never disagree and produce a double image.

Not changed

• Reload from R in frostmod.exe and from Local\FrostModReload (the MXB App's path).
• The Local\FrostModCommand channel (refresh_bike_model) — the app's instant model refresh keeps working.
• The model-swap engine and its FrostMod Models library layout.
• The plugin data callbacks feeding the radar snapshot.

Verification

The preprocessor was emulated over the real file for both switch values:

• FROSTMOD_UI 0 — braces/parens balance; no gated symbol referenced by ungated code; exactly one GetAsyncKeyState in the whole translation unit (the F8 toggle).
• FROSTMOD_UI 1 — balances, 15 key reads, menu branch chain intact. The restore path works.

RadValidateVP and kTrkVisible are now defined but unreferenced in the shipped build — not dangling references, and there's no /W4 or /WX in CMakeLists.txt, so they're silent.

The DLL is MSVC + MinHook + Win32 and can't be built on macOS, so CI on windows-latest is the compile check for this PR.

To check in game

1. Pill appears top-left with the right version.
2. F8 hides / restores it; frostmod.log shows [overlay] pill hidden/shown (F8).
3. An MXB App reload animates the progress bar, then returns to the version text.
4. On track (plugin mode) vs. in menus — same pill, never doubled.

Plan kept in tasks/overlay-version-pill.md.

No DB/schema surface in this repo.

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