test: add comprehensive tests for ReaderEngine

[gvonness-apolitical]

Summary

Adds comprehensive unit tests for the ReaderEngine, testing core orchestration logic.

Tests Added (23)

init() with src

• Load document from URL source
• Load document from File source
• Call onReady callback when loaded
• Call onPageChange callback
• Handle validation errors
• Error when no src or manifest provided

init() with manifest

• Load manifest and first chapter
• Handle invalid format in first chapter

Navigation

• Call next() on reader
• Call prev() on reader
• Do nothing if no reader

Manifest Navigation

• Go to next chapter when at last page
• Go to previous chapter when at first page

Getters

• Return navigation from reader
• Return current format
• Return false for hasManifest when not using manifest
• Return null for manifest controller when not using manifest

Error Handling

• Show error state for DRM protected files
• Show retry button for retryable errors
• Handle unknown errors

updateProgress

• Update loading progress display

destroy

• Clean up all resources
• Destroy manifest controller if present

Impact

• Test count: 191 → 214 (+23 tests)
• New test file: 388 lines

Test plan

• All 214 tests pass
• Build succeeds