Move TuiCode to Terminal.Gui 2.5 and MentalDesk.Tui 0.2.0 #442

Context

TuiCode is on Terminal.Gui 2.1.0. The shared library moved to 2.5 in MentalDesk.Tui 0.2.0 (mentaldesk/tui-style-guide#11), and a-team followed (mentaldesk/a-team#346). So TuiCode can't take any library release until it moves too. Dependabot's #440 makes the version bump and shows what breaks.

#40's paste task also needs 2.5, because bracketed paste arrived in 2.4. That pitch planned the upgrade as part of its fourth task; it's split out here so it can land on its own, as a change that alters nothing.

What 2.5 changes, from the two upgrades already done (the library's Themes.cs and a-team's BundledThemes.cs are worked examples):

• ConfigurationManager is gone, replaced by TuiConfigurationBuilder (migration guide). Bump MentalDesk.Tui and Terminal.Gui #440's build errors are all in DefaultSettingsService.cs. The 2.5 calls are TuiConfigurationBuilder.Shared.RuntimeConfig, ApplyToStaticFacades(), ThemeManager.Theme and ThemeManager.GetThemeNames().
• Config JSON must be nested: Themes and Schemes keyed by name, and no dotted keys such as Dialog.DefaultShadow. 2.5 skips an old-shape file with only a log warning. So src/TuiCode.Workbench/Themes/themes.json needs converting, and anything TuiCode writes to ~/.tui needs checking.
• TextView is obsolete in favour of Terminal.Gui.Editor. EditorTextView derives from it. That only means CS0618 warnings here, since CI doesn't treat warnings as errors.
• A theme switch no longer re-applies key bindings. Check whether WorkbenchHost.NeutralizeBuiltinQuitKey still needs to run after one.
• Tests that fake an open menu need its popover visible as well as enabled before an item takes its letter.
• The real risk is at runtime, where TuiCode reaches into 2.1 internals: the copied draw loop, the content-width cache ([UnsafeAccessor] plus reflection), and the other places AGENTS.md marks "TG 2.1.0".

Acceptance criteria

• TuiCode looks and works as it did: opening and editing files, multiple cursors, undo, soft wrap, find and replace, diff and review tabs, settings, and keybindings.
• Every bundled theme shows as before, and switching between them works.
• A theme and keybindings saved by the current release still load.
• Esc still doesn't quit.

Tests

• Before the version moves, a test for each place TuiCode reaches into Terminal.Gui 2.1 internals, as Make native OS shortcuts (Cmd+C/V/X etc.) work across different terminal emulators #40 planned, so a regression fails a test rather than turning up in use. EditorTextViewDrawTests and EditorTextViewContentWidthTests cover some already.
• Re-diff TextView.Drawing.cs against the editor's copy, as AGENTS.md says to on an upgrade, and drop the copy if 2.5 bounds the loop upstream.
• Theme loading from the converted themes.json: every bundled theme registers and applies.
• CI's AOT smokes (--smoke, --smoke-syntax) on every platform.

Out of scope

• Bracketed paste and the Paste event (Make native OS shortcuts (Cmd+C/V/X etc.) work across different terminal emulators #40).
• Moving the editor off TextView onto Terminal.Gui.Editor.
• Using more of MentalDesk.Tui than TuiCode already does.

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