docs: a verification plan for the native player

Not a generic smoke test. Every case exists because something specific went
wrong, and most were found on hardware after the suites were already green.

The sequences are load-bearing. Two defects this cycle only appeared in a
particular order of actions — play, enable background audio, background,
foreground, exit — and testing the same features in any other order found
neither. So the plan asks for that order explicitly rather than listing
"background audio" as a feature to try.

It also states plainly that a green conformance run is not sufficient evidence
to ship, because both regressions introduced during this work passed
conformance and were caught by a person using the app.

Includes a symptom-to-cause table, because none of these presented as their
cause: a dead play/pause button was an unobserved property, a black screen was
a float that could not become a Duration, and a scrub bar with no scale was a
duration of zero being believed.

"Known open" lists what is deliberately unfixed so each gets a decision rather
than a surprise — device-local resume, the unconfirmed handoff swap, and the
broken side-by-side debug install whose own error message advises an uninstall
that would destroy the real app's data.

docs-site/SUMMARY.md

@@ -48,6 +48,7 @@
 
 - [Build & Release](build/build-release.md)
 - [Release Checklist](release-checklist.md)
+- [Native Player Verification](native-player-verification.md)
 - [Desktop Packaging](build/build-desktop-packages.md)
 - [Windows Build](build/build-windows.md)
 - [Defect Windows](defect-windows.md)