feat(android): add native display and embedded announcements

.project-docs/50-evidence/topics/20260811-native-display-f2456ea9__public-display-api.md

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+# Evidence: Public display API contract
+
+- Date: 2026-08-11
+- Status: Verified
+- Scope: Native Android overview and project-detail implementation
+- Confidence: High
+- Sources: `server/internal/httpapi/public.go`, related Go tests, and live HTTPS responses from `queue.nianxx.cn`
+- Last verified: 2026-08-11
+- Stale trigger: public display routes or DTOs change, or production is deployed from a server revision that differs from this repository
+
+## Confirmed endpoints
+
+- `GET /api/display/overview` returns `{ projects: [...], server_time }` without authentication.
+- `GET /api/display/{identifier}/snapshot` accepts a normalized project code or a 40-128 character display token.
+- Invalid or unknown identifiers return `DISPLAY_NOT_FOUND` with HTTP 404.
+
+## Confirmed overview project fields
+
+- `id`, `code`, `name`, `status`
+- `waiting_count`, `waiting_ticket_count`, `waiting_people_count`
+- `issued_ticket_count`, nullable `latest_ticket_number`, `experienced_people`
+- nullable `current_batch`, `estimated_wait`, `last_updated_at`
+
+## Confirmed snapshot fields
+
+- Project: `project_name`, `status`, nested `project`
+- Queue: `revision`, the three waiting counts, issued/latest/experienced metrics
+- Calls: nullable `current_batch`, `recent_batches`
+- Timing: `estimated_wait`, `server_time`, `last_updated_at`
+
+Each call batch exposes `batch_number`, `sequence`, `status`, `call_mode`, requested/ticket/people counts, `called_at`, and tickets. Each ticket exposes public display number fields, status, and party size; no personal visitor fields are present.
+
+## Implementation consequence
+
+The native detail screen can render and decide announcements from one snapshot request every three seconds. A second announcement-only request is unnecessary. The first successful snapshot after entering or resuming a project must establish a baseline and must not replay the current batch.