Update to TG 2.5.x (PR 5416 preview)

[tig]

⚠️ DO NOT MERGE with the pre-release pin in place

This PR pins a vendored pre-release — Terminal.Gui 2.5.0-tig-remove-cm-followup.2, built from tui-cs/Terminal.Gui#5416 (head e8aba423) and committed as .nupkg/.snupkg files under local_packages/. Before merging:

1. Update the Terminal.Gui pin to the public 2.5.x version in Directory.Build.props (TerminalGuiVersion; the only place the version is pinned — src/Clet/Clet.csproj references $(TerminalGuiVersion)).
2. Delete local_packages/ and remove the tgui-pr-5416 package source from nuget.config (delete the file if nothing else needs it).
3. Revert the .gitignore exception for local_packages/*.nupkg / *.snupkg (the TEMP: block at the end of .gitignore).
4. Re-run build and tests (dotnet restore && dotnet build, then the four test projects) and confirm the restore pulls Terminal.Gui from nuget.org, not a local feed.

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What this PR does

Ports clet to the Terminal.Gui 2.5.0 preview (ConfigurationManager removal). Part of the v2.5.0 ecosystem-validation gate tui-cs/Terminal.Gui#5630; TG changes come from tui-cs/Terminal.Gui#5416. Vendored-package pattern precedent: tig/winprint#272.

API breaks fixed (2.4.5 → 2.5.0 preview)

• ConfigurationManager, ConfigLocations, [ConfigurationProperty], SettingsScope are deleted. Replaced with a new CletConfiguration static wrapper around TuiConfigurationBuilder ("clet"): Apply () at startup (Program.Main) pushes TG settings to the static facades and binds clet's own sections; Reload () re-reads all sources after a config file changes (also reloads TuiConfigurationBuilder.Shared so ThemeManager sees new themes).
• [ConfigurationProperty] reflection discovery is gone. EditorSettings and FileAccessSettings now bind explicitly from the nested "EditorSettings" / "FileAccessSettings" sections (IConfiguration indexer reads — AOT/trim-safe, no binder reflection). The Clet.ConfigTests CM reflection proxies are deleted.
• Config JSON is nested-shape only. Dotted top-level keys ("EditorSettings.LineNumbers", "Key.Separator", …) are detected as legacy and silently skipped by TG 2.5. clet's default config template, the JSONC-preserving EditorSettings.Save, and FileAccessSettings.AddToConfig all now read/write the nested shape ("EditorSettings": { "LineNumbers": … }). Existing user configs with dotted keys are ignored by TG (with a log warning), not migrated.
• ConfigurationManager.ThrowOnJsonErrors / throwing Load+Apply are gone. The MEC loader never throws; per-source errors land in TuiJsonErrors. ConfigClet now: (1) checks JSON syntax with JsonDocument so errors keep line/column info for the editor jump-to-error UX, (2) drains TuiJsonErrors after reload, and (3) explicitly validates the "Theme" value against ThemeManager.GetThemeNames () — the 2.5 loader silently ignores unknown themes, which would have regressed clet's bad-theme error dialog.
• Not hit by clet: View.Text non-virtual (no overrides), IAcceptTarget namespace move (no direct references).

Upstream findings (for the 2.5.0 gate)

1. IApplication.RunAsync deadlocks when called after an await that resumed on an ambient SynchronizationContext (e.g. xunit's) — filed as IApplication.RunAsync deadlocks after await on ambient SynchronizationContext Terminal.Gui#5636. The run loop never completes; the input thread spins forever. await ….ConfigureAwait (false) before RunAsync avoids it; TG 2.4.5 did not have this. Worked around in ConfigClet.RunAsync (commented at the call site). Repro: Clet.IntegrationTests.ConfigCletIntegrationTests.RunAsync_WithBadTheme_StopAfterFirstIteration_ReturnsOk with the ConfigureAwait (false) removed.
2. LinearRange legend rendering regression — filed as LinearRange legend renders stray characters (regression vs 2.4.5) Terminal.Gui#5637: the legend row renders FreenProbrTeam instead of Free Pro Team — stray literal characters drawn with the set-option attribute in the legend spacing cells. Repro: Clet.UITests.CletUiTests.LinearRangeClet_InitialRender_MatchesAnsiGolden (labels Free,Pro,Team).
3. Unknown "Theme" values are silently ignored by the MEC loader (old CM threw). Apps that surfaced bad-theme errors must re-implement validation (clet now does).

Test results (Windows, local; CI runs ubuntu/macos/windows)

Suite	Result
Clet.UnitTests	✅ 314/314
Clet.ConfigTests (rewritten for MEC)	✅ 26/26
Clet.IntegrationTests	✅ 54 (2 pre-existing skips)
Clet.SmokeTests	✅ 14 (1 pre-existing skip)
Clet.UITests (golden renders; not run in CI)	⚠️ 9/19 fail on this box — 5 already fail at baseline (develop + TG 2.4.5, env-dependent goldens); 4 new under 2.5: 3 are benign style diffs (Date/Time/Duration field cursor now underlined instead of inverse) and 1 is the real LinearRange regression above. Goldens intentionally not regenerated — that would bake in an upstream rendering bug and machine-dependent output.

dotnet format --verify-no-changes (CI style gate) and clet --help process smoke: ✅. Release build: ✅ zero warnings.

Follow-up

• Swap to the public Terminal.Gui 2.5.x package and remove the vendored feed (steps above).
• Regenerate the UITests goldens once 2.5.x final ships (cursor-style and checkbox-glyph ☒→☑ changes) and the LinearRange legend regression is fixed upstream.
• Terminal.Gui.Editor (2.5.1) and Terminal.Gui.Cli (0.1.0-develop.7) pins deliberately untouched — those repos are being ported separately.

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