Add Electron and renderer contract tests

[Coldaine]

Origin: docs/todo.md entry dated 2026-04-12
Plan: docs/plans/plan-testing-observability.md (testing layers)

Description

Lock down the preload bridge API so renderer/main don't silently drift. Add tests for main-process replay/export behavior (IPC handlers for shadow:snapshot, shadow:open-replay, shadow:export-replay) and renderer state transitions (busy / error / open / export) so UI regressions are caught without a full e2e.

Acceptance criteria

• Preload bridge API exported as a typed contract; test fails if surface changes unexpectedly
• Main-process replay open path tested with a fixture JSONL
• Main-process export path tested with a temp directory
• Renderer state-machine tests cover busy / error / open / export transitions
• Tests run headlessly in CI (no real Electron BrowserWindow required)