refactor: keep native PDFium navigation

[van-geaux]

Summary

• Keep the native Readium/PDFium navigator as the only PDF rendering and gesture surface.
• Remove the custom spread renderer and custom swipe-axis behavior that made paginated PDFs feel awkward.
• Retain BookOrbit-owned progress, tap zones, volume navigation, reading direction, links, and color inversion.
• Rename the PDF option to Remove gap between pages; it only sets native PDFium page spacing to zero for paginated PDFs.
• Update the PDF testing procedure to describe the native behavior.

Verification

• ./gradlew --no-daemon --console=plain :app:testDebugUnitTest
• ./gradlew --no-daemon --console=plain :app:compileDebugAndroidTestKotlin
• ./gradlew --no-daemon --console=plain :app:lintDebug
• ./gradlew --no-daemon --console=plain :app:assembleDebug
• git -c core.whitespace=cr-at-eol diff --check
• User-confirmed the standard Readium/PDFium behavior works on device.

Artifact

• Debug APK: Lagrange-debug-202610081432.apk
• SHA-256: 23a93791904584844b5144ccd51dfc485485331ca1e81ef9bd9a67a1db5839cb

Scope

• No custom two-page spread pairing.
• No custom vertical landscape pagination.
• No custom swipe-axis filtering.

Closes #193