VS Code extension: navigate from a target in the tree view to its C# definition

Feature idea from the VS Code extension PoC (tree view over the experimental `build-graph.json` export, `FALLOUT001`).

## What

Right-click a target in the **Fallout Targets** tree view (or the build-graph webview) and jump straight to the `Target` property declaration in the consumer''s `Build.cs` / partials — edit the step you''re looking at.

## Implementation sketch

Two complementary resolution strategies, extension-side only (no framework change strictly required):

1. `vscode.executeWorkspaceSymbolProvider` with the target name, filtered to property symbols in the build project — works when C# language services are up.
2. Fallback: regex scan for `Target =>` over `build/**/*.cs` — dumb, fast, no language-service dependency.

Optionally the framework could help: `ExecutableTarget.Member` knows the declaring type and member name, which could be added to `build-graph.json` to disambiguate targets contributed by interfaces/components (e.g. `ITest.Test` vs a local override).

## Context

Part of the broader VS Code-extension exploration (tree view + Mermaid webview PoC, currently local). Filing so the idea survives the PoC phase.