Defer currentLocation updates while the app is inactive

[raphi011]

Upstream ticket: readium#841 (maintainers asked for a ticket to agree on the design before an upstream PR — this draft is the reference implementation).

Follow-ups from the corrected analysis of readium#839 (see also the closed spike #5 — its premise was falsified; the drift was app-side).

Changes

1. EPUBNavigatorViewController no longer adopts currentLocation while the app is inactive.

While the app is backgrounded, iOS renders the app switcher snapshots (running appearance/layout passes for both color schemes) and WebKit may reload or re-lay out the web views. A location computed in that window can reflect a transient scroll position the user never saw; adopting it overwrites currentLocation and misleads any observer persisting the reading progression — the restore family (readium#645, readium#321, readium#309).

The fix gates the debounced updateCurrentLocation on the existing isActive flag, symmetric to needsReloadSpreadsOnActive. Deferred, not dropped: execute(when:) polls until the app is active again and then computes the location once, against the settled layout.

2. Documented the page-start bias of restoring firstVisibleElementLocator().

go(to:) on the captured locator lands on the page on which that element begins — one page back whenever the visible page starts mid-element (e.g. mid-paragraph in a paginated reflowable publication). The protocol doc now spells out when to prefer it (page grid changed) vs currentLocation (layout unchanged).

Testing

• Full package suite on iOS Simulator: xcodebuild test -scheme Readium-Package — 101 tests, 28 suites, green.
• The lifecycle path has no unit harness (same as needsReloadSpreadsOnActive).
• Host-app pass: a reader app pinning this fix (cherry-picked onto its integration branch) runs its full suite green, and the reader smoke passes — open at persisted position, background→foreground stays on page, page turns work.

Draft: intended to iterate here first, then propose upstream once readium#841 settles the design.

🤖 Generated with Claude Code

[raphi011]

Host-app pass done: Kios now pins this branch content (the fix commit cherry-picked onto its integration branch as aca293e03). Full app test suite green against the new pin, and the reader smoke passes on the iOS Simulator — open at persisted position, background→foreground stays on the same page, edge-tap page turn works. The remaining verification item from the description is covered.