docs: establish integrated project memory

.project-docs/10-decisions/decision-index.md

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+# Decision Index
+
+## Active Decisions
+
+| ID | Decision | Status | Date | Applies To | Detail |
+|---|---|---|---|---|---|
+| ADR-001 | Use native Android Views for the dedicated large-screen app | Accepted | 2026-08-12 | `android/` | [ADR](adr-001-native-android-display.md) |
+| ADR-002 | Compose embedded Mandarin WAV fragments and stream them through `AudioTrack` | Accepted | 2026-08-12 | Android project-screen announcements | [ADR](adr-002-embedded-audio-announcements.md) |
+
+## Superseded Decisions
+
+| ID | Decision | Superseded By | Date |
+|---|---|---|---|
+
+## Decision Criteria
+
+Create or update an ADR when a choice affects:
+
+- project positioning
+- architecture boundaries
+- public behavior
+- data model
+- long-term maintenance
+- user-facing workflow