Set FPGA top entity from the project explorer and restore the top entity indicator

[Copilot]

Since the top entity rework, the top entity could only be set through project settings, and the file containing it was no longer visually identifiable. This adds a context-menu shortcut to set the top entity and reinstates the indicator overlay with a new meaning.

Changes

• UniversalFpgaProjectManager — HDL files (.vhd, .vhdl, .v, .sv) now expose a Set Top Entity submenu with one entry per entity/module declared in the file (extracted via the file's INodeProvider). Selecting one sets topEntity and saves; the active entity is marked with a check icon.
• Top entity indicator — re-added the overlay, now marking the file that contains the top entity rather than the entity itself, matching the name-based topEntity model.
• UniversalFpgaProjectRoot.TopEntityFilePath — new runtime-only (non-persisted) cache resolving topEntity → owning file via FpgaService.GetAllTopEntitiesAsync. The manager recomputes it on project load/reload and subscribes to topEntity changes, so the indicator stays correct regardless of where the top entity is changed (context menu, settings, pin planner).
• UniversalProjectRoot.InvalidateAllModifications() — reusable helper to re-run entry modification handlers across all loaded entries when project-wide state changes.
• Docs — corrected a stale topEntityFile reference and documented the new behavior.

Notes for reviewers

• The submenu is populated asynchronously (entity extraction is async); the menu item is created with an empty ObservableCollection and filled on the UI thread once parsing completes.
• The indicator does not refresh when a top-entity file is later toggled as a test bench / compile-excluded (only topEntity changes and reloads trigger recomputation) — an intentional edge-case trade-off to avoid re-scanning on unrelated property changes.