fix(phone): stop in-place reload from writing progress under the next episode

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Bug and impact

During an in-place phone player reload (auto-advance, On Deck, version switch), backing out while the next item is still loading could persist the previous episode's timestamp under the next episode's content id.

Concrete trigger: watch episode A, auto-play starts loading episode B, hit Back during the loading spinner. Episode B then resumes at episode A's position.

Root cause

publishLoadingState retargets contentId immediately but leaves position and versions on the previous item until Ready. onExit submitted finalPlaybackPositionWriter using that mix of identities.

onPositionChanged already ignores samples while positionReportsBlockedForPendingLoad is set. PR #241 guards the matching hole in onPlayingChanged. This is the remaining durable-write sink on the same transition.

Fix

Remember the last Ready (contentId, fileId) and, when a load is still pending, attribute the exit write to that identity instead of the loading target. Same-content recovery still writes the previous item correctly rather than dropping the snapshot.

Validation

• PlayerViewModelLoadOwnershipIntegrationTest.exitDuringInPlaceReloadWritesPreviousItemNotTheLoadingTarget
• PlayerViewModelLoadOwnershipIntegrationTest.exitAfterReadyStillWritesCurrentProgress
• MobilePlayerLifecyclePerformanceSourceTest (teardown source contract still holds)

Test plan

• Auto-advance to the next episode and press Back during the loading spinner; the next episode must not inherit the previous timestamp
• Watch a title to a non-zero position and leave the player normally; progress still saves
• Version switch, then Back during reload; the previous version keeps its last position