Fix explicit active file drift in multi-file bridge

[StealthEyeLLC]

Summary

Fixes #72.

This makes explicit multi-file targeting sticky in the local WebSocket Desktop Bridge. Before an MCP client explicitly targets a file, the legacy behavior is preserved: the active file follows the latest connected/user-touched file. After setActiveFile() is called (normally through figma_navigate), passive peer FILE_INFO, SELECTION_CHANGE, and PAGE_CHANGE events continue updating per-file state, but no longer silently retarget subsequent tool calls.

Why

In multi-file sessions, background events from another Figma file can currently override an explicit figma_navigate() target. That makes long-running agent workflows unsafe: an operation can begin in file A and silently continue in file B after a tab focus, selection event, page event, or plugin reconnect.

Changes

• Add an explicit active-file mode to FigmaWebSocketServer.
• Preserve implicit auto-switching until a caller explicitly chooses a target.
• Prevent peer selection/page events from stealing an explicit active target.
• Prevent peer FILE_INFO reconnects/new file connections from stealing an explicit active target.
• Keep same-file reconnects working without clearing the explicit target.
• Reset explicit mode only when the active file is removed/disconnected and the server falls back to another connected file.
• Make the figma_navigate already-connected path call setActiveFile() so it pins the target explicitly even when the requested file is already active.
• Add regression coverage for peer SELECTION_CHANGE, PAGE_CHANGE, and FILE_INFO reconnect/new-file drift.

Validation

Run on OVH using the upstream repo checkout on branch fix-explicit-active-file-lock.

• npm ci --include=dev ✅
• npx -y node@22 node_modules/jest/bin/jest.js tests/websocket-bridge.test.ts --runInBand ✅
• npx -y node@22 node_modules/typescript/bin/tsc -p tsconfig.local.json --noEmit ✅
• npx -y node@22 node_modules/typescript/bin/tsc --project tsconfig.local.json ✅
• npx -y node@22 node_modules/jest/bin/jest.js --runInBand ✅ — 45 suites / 1339 tests passed
• git diff --check ✅

Note: full tsc --noEmit still reports existing app UI type errors in src/apps/*/ui/mcp-app.ts unrelated to this change. The touched local-server path passes tsconfig.local.json type-check and build, and the full Jest suite passes under Node 22.