Cover Android custom-UI APIs in navigation AGENTS.md

[mattpodwysocki]

Split out of #80.

Why

README.md:801 tells users to curl a skill's AGENTS.md into their project root for Codex and Copilot — it ships standalone. This one was silent on four Android Navigation SDK areas that references/ covers in detail.

Silence in a condensed file is not neutral. Asked about them, the model filled the gaps by inventing APIs:

• named upcomingRouteAlerts instead of RouteProgress.upcomingRoadObjects
• inverted addUpcomingManeuverArrow and updateUpcomingManeuverArrow
• said calling MapboxAudioGuidance.create() directly is wrong, when it is the correct call if you manage the instance lifecycle yourself

What was added

Compact coverage of each, drawn from references/android-navigation-sdk.md and references/android-performance-antipatterns.md — 60 lines, no new guidance:

• Route line via MapboxRouteLineApi/MapboxRouteLineView, with getAlternativeMetadataFor and the cancel() teardown neither object performs on its own
• Maneuver arrows as a separate API/View pair, recomputed on every progress update rather than drawn once
• MapboxAudioGuidance.getRegisteredInstance(), which self-registers with the MapboxNavigationApp lifecycle, and when create() is right instead
• RouteProgress.upcomingRoadObjects — already filtered and ordered — versus re-filtering NavigationRoute.upcomingRoadObjects by position
• Observer registration through onAttached/onDetached, since an inline lambda with no stored reference can never be unregistered
• Why reading route data under nativeRouteObject(true) is expensive

Measured

Agents surface at --repeats=5. The four affected evals were at 0/9, 5/12, 6/12 and 8/12; the skill moved from 82% to 97%.

Test plan

• npm run check passes
• mapbox-navigation-patterns re-run at --repeats=5 on both surfaces

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