Consolidate createFakeBackend factories into a shared test helper

[ThomasK33]

Spun off from #78 review (DEREM-11).

Context

There are now four hand-rolled createFakeBackend() factories implementing
RendererBackend across the test suite:

• test/unit/renderer/registry.test.ts (predates refactor: share screenshot capture and artifact persistence #78)
• test/unit/host/renderer.test.ts (predates refactor: share screenshot capture and artifact persistence #78)
• test/unit/renderer/libghosttyVtBackend.test.ts (createFallbackBackend,
  predates refactor: share screenshot capture and artifact persistence #78)
• test/unit/screenshot/capture.test.ts (introduced in refactor: share screenshot capture and artifact persistence #78, the most
  feature-rich variant: supports onScreenshot, resultOverrides,
  writePng, and injectable failures)

Each copy hand-writes the same replayTo/snapshot/getVisibleText/
dispose stubs. The screenshot copy is currently the most flexible and is
a good basis for consolidation.

Acceptance ideas

• Introduce test/helpers/fakeBackend.ts exporting createFakeBackend(...)
  with the union of capabilities the existing copies need (configurable
  return values, optional callbacks, optional renderer-failure injection).
• Migrate all four call sites to the shared helper.
• Keep test names and behaviour unchanged.

Out of scope here

• Renderer behaviour or production code changes.

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Filed automatically as part of the #78 review cycle.

[ThomasK33]

This was generated by AI during triage.

Agent Brief

Category: enhancement
Summary: Consolidate four hand-rolled createFakeBackend() / createFallbackBackend() factories into a single shared test helper with the union of all capabilities.

Current behavior:
Four separate test files each define their own RendererBackend fake factory, all implementing the same interface with near-identical replayTo/snapshot/screenshot/getVisibleText/dispose stubs. The factories differ only in which optional capabilities they support:

1. registry tests — minimal: configurable rendererBackend string, default stubs, no failure injection.
2. host/renderer tests — adds configurable bootImplementation injection, a setBooted() mutator, and exposes individual mock handles (bootMock, replayToMock, snapshotMock, screenshotMock, getVisibleTextMock, disposeMock).
3. libghosttyVtBackend tests (createFallbackBackend) — exposes bootMock, replayToMock, screenshotMock, disposeMock. Tracks booted state internally. Returns slightly different default values for screenshot fields (capturedAtSeq: 2, cols: 12, rows: 5, pngSizeBytes: 123).
4. screenshot/capture tests — the most feature-rich: onScreenshot callback, resultOverrides for overriding any ScreenshotResult field, writePng flag for actually writing PNG bytes to disk, fail option for injecting errors.

Desired behavior:
A single shared createFakeBackend() helper that covers the union of all four variants' capabilities. All four call sites should import from the shared helper. Existing test names, assertions, and behavior must remain unchanged.

Key interfaces:

• RendererBackend (from src/renderer/backend.ts) — the interface all fakes implement. Methods: boot(), replayTo(input), snapshot(options?), screenshot(outputPath, options?), getVisibleText(), dispose(). Properties: rendererBackend (string), isBooted (boolean).
• The shared helper should accept an options object supporting at minimum:
  • rendererBackend?: string — backend identifier (default: a sensible test value)
  • bootImplementation?: () => Promise<void> — custom boot behavior
  • onScreenshot?: (outputPath: string, options?: ScreenshotOptions) => void — spy callback
  • resultOverrides?: Partial<ScreenshotResult> — override any screenshot result field
  • writePng?: boolean — write real PNG bytes to disk (default: false or true depending on context)
  • fail?: Error — make screenshot() throw
• The return type should expose individual mock handles (e.g., bootMock, replayToMock, etc.) and a setBooted(value: boolean) mutator, so tests that inspect or control internal state can do so.

Acceptance criteria:

• A shared createFakeBackend() function exists (either in test/helpers.ts or test/helpers/fakeBackend.ts)
• All four original factory definitions are removed and replaced with imports from the shared helper
• The registry tests, host/renderer tests, libghosttyVtBackend tests, and screenshot/capture tests all pass with no changes to test names or assertions
• The shared helper supports all capabilities listed above (boot injection, mock handle exposure, setBooted, onScreenshot, resultOverrides, writePng, fail)
• vitest run passes with no regressions

Out of scope:

• Renderer production code changes
• New test cases or changing what existing tests verify
• Refactoring beyond the four factory consolidation