Merge branch 'main' into source-routing-packet-format

docs/GeohashPresenceSpec.md

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+# Geohash Presence Specification
+
+## Overview
+
+The Geohash Presence feature provides a mechanism to track online participants in geohash-based location channels. It uses a dedicated ephemeral Nostr event kind to broadcast "heartbeats," ensuring accurate and privacy-preserving online counts.
+
+## Nostr Protocol
+
+### Event Kind
+A new ephemeral event kind is defined for presence heartbeats:
+- **Kind:** `20001` (`GEOHASH_PRESENCE`)
+- **Type:** Ephemeral (not stored by relays long-term)
+
+### Event Structure
+The presence event mimics the structure of a geohash chat message (Kind 20000) but without content or nickname metadata, to minimize overhead and focus purely on "liveness".
+
+```json
+{
+  "kind": 20001,
+  "created_at": <timestamp>,
+  "tags": [
+    ["g", "<geohash>"]
+  ],
+  "content": "",
+  "pubkey": "<geohash_derived_pubkey>",
+  "id": "<event_id>",
+  "sig": "<signature>"
+}
+```
+
+*   **`content`**: Must be empty string.
+*   **`tags`**: Must include `["g", "<geohash>"]`. Should NOT include `["n", "<nickname>"]`.
+*   **`pubkey`**: The ephemeral identity derived specifically for this geohash (same as used for chat messages).
+
+## Client Behavior
+
+### 1. Broadcasting Presence
+
+Clients MUST broadcast a Kind 20001 presence event globally when the app is open, regardless of which screen the user is viewing.
+
+*   **Global Heartbeat:**
+    *   **Trigger:** Application start / initialization, or whenever location (available geohashes) changes.
+    *   **Frequency:** Randomized loop interval between **40s and 80s** (average 60s).
+    *   **Scope:** Sent to *all* geohash channels corresponding to the device's *current physical location*.
+    *   **Privacy Restriction:** Presence MUST ONLY be broadcast to low-precision geohash levels to protect user privacy. Specifically:
+        *   **Allowed:** `REGION` (precision 2), `PROVINCE` (precision 4), `CITY` (precision 5).
+        *   **Denied:** `NEIGHBORHOOD` (precision 6), `BLOCK` (precision 7), `BUILDING` (precision 8+).
+    *   **Decorrelation:** Individual broadcasts within a heartbeat loop must be separated by random delays (e.g., 2-5 seconds) to prevent temporal correlation of public keys across different geohash levels. The main loop delay is adjusted to maintain the target average cadence.
+
+### 2. Subscribing to Presence
+
+Clients must update their Nostr filters to listen for both chat and presence events on geohash channels.
+
+*   **Filter:**
+    *   `kinds`: `[20000, 20001]`
+    *   `#g`: `["<geohash>"]`
+
+### 3. Participant Counting
+
+The "online participants" count shown in the UI aggregates unique public keys from both presence heartbeats and active chat messages.
+
+*   **Logic:**
+    *   Maintain a map of `pubkey -> last_seen_timestamp` for each geohash.
+    *   Update `last_seen_timestamp` upon receiving a valid **Kind 20001 (Presence)** OR **Kind 20000 (Chat)** event.
+    *   A participant is considered "online" if their `last_seen_timestamp` is within the last **5 minutes**.
+
+### 4. UI Presentation
+
+The presentation of the participant count depends on the geohash precision level and data availability.
+
+*   **Standard Display:** For channels where presence is broadcast (Region, Province, City) OR any channel where at least one participant has been detected, show the exact count: `[N people]`.
+*   **High-Precision Uncertainty:** For high-precision channels (Neighborhood, Block, Building) where:
+    *   Presence broadcasting is disabled (privacy restriction).
+    *   **AND** the detected participant count is `0`.
+    *   **Display:** `[? people]`
+    *   **Reasoning:** Since clients don't announce themselves in these channels, a count of "0" is misleading (people could be lurking).
+
+### 5. Implementation Details (Android Reference)
+
+*   **`NostrKind.GEOHASH_PRESENCE`**: Added constant `20001`.
+*   **`NostrProtocol.createGeohashPresenceEvent`**: Helper to generate the event.
+*   **`GeohashViewModel`**:
+    *   `startGlobalPresenceHeartbeat()`: Coroutine that `collectLatest` on `LocationChannelManager.availableChannels`.
+    *   Implements randomized loop logic (40-80s) and per-broadcast random delays (2-5s).
+    *   Filters channels by `precision <= 5` before broadcasting.
+*   **`GeohashMessageHandler`**:
+    *   Refactored `onEvent` to update participant counts for both Kind 20000 and 20001.
+*   **`LocationChannelsSheet`**:
+    *   Implements the `[? people]` display logic for high-precision, zero-count channels.
+
+## Benefits
+
+*   **Accuracy:** Counts reflect both active listeners (via heartbeats) and active speakers (via messages).
+*   **Privacy:** High-precision location presence is NOT broadcast. Temporal correlation between different levels is obfuscated via random delays.
+*   **Consistency:** "Online" status is maintained globally while the app is open.
+*   **Transparency:** The UI correctly reflects uncertainty (`?`) when privacy rules prevent accurate passive counting.