Fix workbench theme switcher against current and upcoming SharpConsoleUI

[nickprotop]

Summary

The Chronicle Workbench theme switcher built themes by concrete type
(new ClassicTheme(), new ModernGrayTheme()) and by reflective type
name (SharpConsoleUI.Plugins.DeveloperTools.DevDarkTheme). The upcoming
SharpConsoleUI release removes ClassicTheme, moves DevDarkTheme out of
the library, and replaces the static ThemeRegistry with a per-instance
windowSystem.ThemeRegistryService — so the current code stops compiling
against it.

This resolves the theme API at runtime through a small reflection bridge,
so the same source compiles and runs against both the current package
and the next release. The package reference is unchanged.

Changed

• Theme switching now goes through a new WorkbenchThemes helper that uses
  the version-stable ThemeStateService.SwitchTheme(name) and resolves the
  theme catalogue from whichever registry the running version exposes
  (per-instance ThemeRegistryService, falling back to the static
  ThemeRegistry) via reflection.
• The F9 / F10 / F11 primary themes are resolved per version: the current
  package keeps the original Modern Gray / Classic / Dev Dark; the next
  release maps to Modern Gray / Forest / Crimson (Classic and Dev Dark
  no longer exist there). The Help menu gains a More Themes submenu
  listing every registered theme, and the keyboard-shortcuts overlay
  reflects the active primary themes.

---

Note: the reflection bridge in WorkbenchThemes is intentionally
temporary — it exists only to span the 2.4.78 → 2.4.79 transition. Once
the SharpConsoleUI dependency is pinned to 2.4.79+ and 2.4.78 is no
longer supported, it can be simplified to direct ThemeRegistryService
calls and the static-registry / Dev Dark fallbacks removed.

Note: I could not produce a full local build — this repo's source
generators require a newer Roslyn than my local SDK ships (CS9057), so
generator-emitted symbols are unavailable in my environment (this also
affects a clean upstream/main checkout, independent of this change).
The theme files were verified to compile against both the current
SharpConsoleUI package and a build of the next release, and the runtime
version-resolution (primary slots and registry enumeration) was tested
against both. CI with the project's toolchain should build normally.