Android: TalkBack accessibility support

[danielraffel]

Problem

Pulp has an accessibility interface layer (core/view/accessibility) but it isn't wired to Android's TalkBack. The iOS VoiceOver bridge exists; Android needs the equivalent.

Proposed

Implement an `AccessibilityNodeProvider` (or AccessibilityDelegate) on the Android Pulp surface view that maps Pulp widgets' accessibility interfaces (Value, Text, Button, Slider, etc.) to Android AccessibilityNodeInfo.

Acceptance

• TalkBack announces focused Pulp widgets with role + value
• Swipe gestures move focus between Pulp widgets
• Value changes are announced
• Tests cover the node provider API on a headless Android run

[danielraffel]

Implementation plan written: planning/android-talkback-accessibility-87.md (in the pulp-planning submodule). Approach: AccessibilityNodeProvider attached to PulpSurfaceView, JNI bridge into the existing core/view/accessibility* interfaces, per-widget role/value/label population, value-change announcements via sendAccessibilityEvent(TYPE_VIEW_TEXT_CHANGED). Flatten to top-level interactive widgets first; deeper hierarchy later if needed.