fix(android): serialize EPUB navigation after initial page load

[ddfreiling]

Calling goToLocator immediately after mounting an Android EPUB navigator could return success and then be overwritten by the navigator's initial restore, leaving the reader on the first reading-order item. Navigation now waits for the first visible page to load before applying the requested locator.

The regression test reproduces the race by navigating to the second reading-order item immediately after mounting the reader and waiting for the emitted locator to settle there.

Validation:

• Baseline regression test timed out after 60 seconds and remained on the first item.
• All 10 Android navigation integration tests passed with the fix.
• Kotlin compilation and plugin unit tests passed.
• ktlint --format, bin/format, bin/analyze, and git diff --check passed.
• The aggregate Gradle unit-test task remains red because the unrelated wakelock_plus test task cannot start its JUnit Platform executor; :flutter_readium:testDebugUnitTest passes.