Workspace layer: Multi-roster management with compilation tracking

[amis92]

Goal

Introduce a WhamWorkspace that manages the current state of catalogues and rosters, provides lazy per-roster compilation tracking, document identity, and change notifications for UI consumers. Analogous to Roslyn's Workspace / Solution / CompilationTracker infrastructure.

Parent epic: #279
Depends on: #280 (SymbolKey), #281 (Incremental compilation)

Problem

Currently there is no centralized state management above RosterEditor:

• RosterEditor manages a single roster with undo/redo
• No concept of "the workspace" containing catalogues + multiple rosters
• No document identity tracking (which source trees changed between versions)
• No lazy compilation rebuilding — everything is eager
• No change notification mechanism for UI consumers to react to state changes

Design

Core Components

WhamWorkspace
├── Loaded catalogues (shared CatalogueCompilation from Phase 2)
├── Per-roster state:
│   ├── CompilationTracker (lazily rebuilds RosterCompilation)
│   ├── RosterEditor (undo/redo stack)
│   └── Document identity (source tree tracking)
├── Change notifications for UI
└── Background diagnostics support

WhamWorkspace Responsibilities

• Catalogue management: Load/unload catalogues, maintain shared CatalogueCompilation
• Roster management: Open/close rosters, each with its own CompilationTracker
• Compilation tracking: Lazily rebuild roster compilations when source trees change
• Document identity: Track which source trees (catalogues, rosters) are loaded, map old↔new across versions
• Change notifications: Event-based notification when compilations change (for UI refresh, background diagnostics)

CompilationTracker (Per-Roster)

Analogous to Roslyn's RegularCompilationTracker:

internal class CompilationTracker
{
    // Lazily computed compilation, cached until invalidated
    private Lazy<RosterCompilation> lazyCompilation;
    
    // Queue of pending changes (roster tree replacements)
    private ImmutableList<TranslationAction> pendingActions;
    
    // Get or compute the current compilation
    public RosterCompilation GetCompilation();
    
    // Fork: create a new tracker with a pending change
    public CompilationTracker WithRosterTreeChange(SourceTree newTree);
}

Roslyn Analogy

Roslyn	Wham
Workspace	WhamWorkspace
Solution	Immutable snapshot of catalogues + rosters
Project	Catalogue or Roster
Document	SourceTree
CompilationTracker	Per-roster CompilationTracker
TranslationAction	Roster tree changes
DocumentId	Source tree identity

Tasks

• Design WhamWorkspace API (load catalogues, open/close rosters)
• Implement CompilationTracker for lazy per-roster compilation management
• Implement document identity tracking for source trees
• Add change notification events (CompilationChanged, RosterStateChanged)
• Integrate with RosterEditor — workspace-aware editor operations
• Add support for background diagnostics (compile + validate on background thread)
• Add tests for workspace lifecycle (load, edit, close)
• Add tests for multi-roster workspace scenarios
• Add tests for change notification ordering and consistency