limoni-mcp MCP server

### Breaking

The API review before v1.0: one name for each thing, and the frame's
internals out of the public surface. Every removal has a replacement that
already existed; nothing here changes what an application can do.

- Renamed: `graphics.GetCachedEscapeSequence` → `CachedEscapeSequence`,
  `graphics.GetImageID` → `ImageID`, `driver.GetPlatformCapabilities` →
  `PlatformCapabilities`, `VirtualDataState.GetCachedRowText` →
  `CachedRowText`, `accessibility.AccessibilityNode` → `accessibility.Node`
  (the old name stays one release as a deprecated alias),
  `layout.LayoutDiagnostic` → `layout.Diagnostic`,
  `layout.LegacyMeasurable` → `layout.SizeHinter`, and
  `CommandPalette.SelStyle` → `SelectedStyle`, as in every other widget.
- Removed duplicates: `limoni.HAlignLeft/Center/Right` (use
  `AlignLeft/Center/Right`), `limoni.Layout` (`FlexLayout`),
  `driver.Driver` and `limoni.Driver` (`driver.Backend`),
  `Terminal.Backend()` (`Driver()`), `Program.SendBackend` (`SendDriver`),
  `engine.MessageFromBackend` (`MessageFromDriver`),
  `Frame.RenderComponent` (`RenderWidget`).
- Removed the old modal API: `Frame.ActiveModal` and the `Modal` type
  mirrored the topmost modal layer. `RegisterModal` still works; read
  `Frame.TopmostModal()` instead of the field.
- `Frame` no longer exports its bookkeeping: `ImageRegions`,
  `DebugRegions`, `WidgetStats`, `Accessibility`, `Theme`, `ThemeSet` and
  `EventRegions` (whose element type was already unexported) are private.
  Use `RegisterImageRegion` to place a picture (new; or, as pikap now
  does, a widget calling `ctx.RegisterImage`, which works on both sides),
  `ImageRegionsSnapshot`, `AccessibilityTree` and `SetTheme`. `Buffer`,
  `FocusManager`, `ClickRegions`, `Layers`, `Hyperlinks` and
  `ImageProtocol` stay, because applications and testkit read them.

### Added

- `Frame.BeginArea(r)` / `EndArea()`: a view written for the whole
  screen drawn in part of it — a tab, a pane, a preview. Between the two,
  `f.Area()` is `r`, nothing is drawn outside it, and click, scroll and
  event regions are cut to it, so a pane cannot take its neighbour's clicks.
  Areas nest; one left open is closed after the frame. Underneath,
  `Buffer.SetClip` / `ClearClip` / `Clip` limit every write, `Get` included.
  Modals and layers are not scoped to an area yet. Asked for by
  residual's tabbed multiplexer. Writes check the clip, which costs 1–4% in
  the widget draw benchmarks (0 allocations, unchanged).
- `Frame.RegisterImageRegion(region)`: place a picture with its own
  z-index, for applications that draw their own pixels.

### Changed

- `Buffer.Snapshot` reads with `CellAt`, so taking one no longer marks the
  buffer dirty, and works while a clip is set.
- A focused `Button` with neither `Style` nor `FocusedStyle` set takes the
  frame theme's `focus` role; explicit styles, and frames without a theme,
  keep reverse video and bold (#100, by @gezerui888).

### Fixed

- Windows Terminal and the Windows console got 16 colours: Windows Terminal
  sets neither `COLORTERM` nor `TERM`, only `WT_SESSION`. `WT_SESSION`, or a
  Windows process with no terminal named in `TERM`/`TERM_PROGRAM`, now means
  truecolor.
- Apple's Terminal: 24-bit colour only from build 470 (older builds read
  `38;2;r;g;b` as separate attributes and drew reverse video and blue); no
  `REP` (a repeated `─` drew once and the row landed short) and no scroll
  regions (rows the diff thought had moved stayed where they were).
- `go install …/apps/termimg@latest` on Windows stopped at a dozen compile
  errors (`unix.Poll`, `Setsid`, `SIGWINCH`). termimg is Unix-only now by
  build tag; on Windows it builds and says so, and that Windows Terminal
  1.22 and later show sixel by themselves.

- `apps/termimg` made large output 23 times slower in Alacritty: `cat` of
  100 MB took 26 s instead of 1.1 s. The shadow screen (`x/vt`) copies the
  whole screen for every line it scrolls. Lines that would only scroll
  through it are now ski