Implemented history writer

Docs/StreamHub-UserGuide.md

@@ -98,7 +98,78 @@ client and uses full-rate ring data (not the decimated live stream).
 On capture, the trigger view shows all plotted signals relative to the trigger
 instant (marker at t = 0) over `[-pre, +post]`.
 
-## 5. Troubleshooting
+## 5. History (disk-backed storage)
+
+StreamHub can store signal data to disk, allowing you to browse hours of past
+data even after the in-memory ring buffers have wrapped.
+
+### Enabling history
+
+Add a `+History` block to the hub configuration:
+
+```
+Hub = {
+    WSPort = 8090
+    +History = {
+        Directory      = "/data/streamhub_history"
+        DurationHours  = 1
+        Decimation     = 10
+        FlushIntervalSec = 5
+        MinDiskFreeMB  = 200
+    }
+    Sources = { … }
+}
+```
+
+| Key | Default | Description |
+|-----|---------|-------------|
+| `Directory` | *(required)* | Root directory for `.shist` files. Created automatically. Each source gets a subdirectory; each signal a separate file. |
+| `DurationHours` | 1 | How many hours of data to keep per signal. The file size is fixed at creation: `duration × samplingRate / decimation × 16 bytes`. |
+| `Decimation` | 1 | Write every N-th sample (1 = no decimation). Useful for high-rate signals where full-rate disk writes are impractical. |
+| `FlushIntervalSec` | 5 | How often file headers are flushed to disk (data is written immediately via `pwrite`). |
+| `MinDiskFreeMB` | 500 | Writing pauses when free disk space drops below this threshold and resumes when space is available again. |
+
+### Disk usage
+
+Each sample on disk is 16 bytes (8 bytes timestamp + 8 bytes value). Example:
+
+- 10 signals at 1 ksps, decimation 10, duration 1 hour:
+  `10 × (1000/10) × 3600 × 16 bytes = 57.6 MB`
+- 10 signals at 1 Msps, decimation 1000, duration 1 hour:
+  `10 × (1e6/1000) × 3600 × 16 bytes = 576 MB`
+
+Files are pre-allocated at creation and never grow. The directory structure is:
+
+```
+/data/streamhub_history/
+├── scalar/
+│   ├── Sine1.shist
+│   └── Sine2.shist
+├── med/
+│   ├── Ch1.shist
+│   └── Ch2.shist
+└── fast/
+    ├── Ch3.shist
+    └── Ch4.shist
+```
+
+### Using history in clients
+
+When history is enabled, both the web SPA and ImGui client show a **HIST**
+indicator. History data is fetched transparently:
+
+- **Zoom**: when you zoom or pan into a time range that extends beyond the
+  in-memory ring buffer, the client issues a `historyZoom` request in parallel
+  with the regular `zoom` request and merges the results — history data fills
+  the older part of the window, ring data the recent part.
+- **Status**: the web SPA displays a history badge in the status bar showing
+  the configured duration and decimation factor. The ImGui client shows
+  "HIST" in the plot header when history data is being displayed.
+
+History files survive hub restarts. If a `.shist` file exists with a matching
+capacity, the hub reopens it and continues writing where it left off.
+
+## 6. Troubleshooting
 
 | Symptom | Check |
 |---------|-------|
@@ -111,7 +182,7 @@ instant (marker at t = 0) over `[-pre, +post]`.
 Hub-side logs go to stdout; the demo scripts write
 `/tmp/streamhub_e2e_{marte,hub}.log`.
 
-## 6. Further reading
+## 7. Further reading
 
 - [StreamHub-API.md](StreamHub-API.md) — WebSocket protocol (write your own client)
 - [StreamHub-Developer.md](StreamHub-Developer.md) — internals, build, tests