Implemented history writer

Docs/StreamHub-Developer.md

@@ -21,6 +21,7 @@ Wire protocols: [Protocol.md](Protocol.md) (UDPS, source → hub) and
 | `SignalRingBuffer.h` | Per-signal (t,v) ring: monotonic write counter, `ReadSince` cursor reads, binary-search `ReadRange` |
 | `TriggerEngine.{h,cpp}` | Trigger FSM (IDLE/ARMED/COLLECTING/TRIGGERED), edge detection per decoded sample |
 | `UDPSourceStats.h` | 512-entry cycle/frag/byte rings → avg/std/min/max, rate, 20-bin histogram |
+| `HistoryWriter.{h,cpp}` | Disk-backed circular history: per-signal `.shist` files, `WriteTick`, `ReadRange` (binary search via `pread`), disk space monitoring |
 | `WSServer.{h,cpp}` | RFC 6455 server: handshake, framing, per-client write mutex, broadcast/unicast |
 | `WSFrame.h`, `SHA1.h`, `Base64.h` | Header-only WS plumbing, shared with the ImGui client |
 | `LTTB.h` | Largest-Triangle-Three-Buckets decimation |
@@ -124,6 +125,66 @@ Sources = {
 Sources from `SourcesFile` are loaded after the static `Sources` block;
 sources added at runtime via WS `addSource` get ids `s1, s2, …`.
 
+### History configuration
+
+An optional `+History` block enables disk-backed circular storage
+(`HistoryWriter`). The `+` prefix is MARTe2 `StandardParser` syntax for a
+child node; the hub looks for both `+History` and `History` as the node name.
+
+```
++History = {
+    Directory      = "/data/streamhub_history"   // required
+    DurationHours  = 1          // hours of data to retain per signal (default 1)
+    Decimation     = 10         // keep every Nth sample (default 1)
+    FlushIntervalSec = 5        // header flush period in seconds (default 5)
+    MinDiskFreeMB  = 500        // pause writes below this threshold (default 500)
+}
+```
+
+Per-signal file capacity is computed at source CONFIG time:
+`capacity = ceil(DurationHours × 3600 × samplingRate / Decimation)`, minimum
+1000 pairs.
+
+### `.shist` binary file format
+
+Each signal gets one file: `<Directory>/<sourceId>/<signalName>.shist`.
+The file size is fixed at creation (`64 + capacity × 16` bytes) and never grows.
+
+```
+Offset  Size  Field
+0       4     Magic: "SHR1"
+4       4     uint32 version (1)
+8       4     uint32 capacity (max pairs)
+12      4     uint32 head (next write position, 0-based, wraps at capacity)
+16      4     uint32 count (valid entries, ≤ capacity)
+20      4     uint32 decimation
+24      8     float64 tOldest (Unix seconds)
+32      8     float64 tNewest (Unix seconds)
+40      24    reserved (zero-padded to 64 bytes)
+64      …     data: capacity × 16 bytes (float64 time + float64 value per pair)
+```
+
+Data is written at the `head` position and wraps circularly. The oldest valid
+entry is at logical index `(head + capacity − count) % capacity`. All I/O
+uses `pwrite`/`pread` (no `mmap`), so concurrent reads from WS threads are
+safe without locking.
+
+Headers are flushed to disk every `FlushIntervalSec` seconds and on shutdown.
+On restart, if an existing file has matching magic, version and capacity, it is
+reopened — head/count/time bounds are restored from the on-disk header.
+
+### History query path
+
+`historyZoom` requests (see [StreamHub-API.md](StreamHub-API.md)) call
+`HistoryWriter::ReadRange` which performs binary search over the circular file
+using `pread` to locate the `[t0, t1]` window, then copies matching pairs.
+If the result exceeds the requested `n`, LTTB decimation is applied (same
+`LTTBDecimate` as in-memory zoom).
+
+Both the web SPA and ImGui client issue `historyZoom` in parallel with regular
+`zoom` and merge the results: history covers the older part of the visible
+window, the in-memory ring covers the recent part.
+
 ## 7. Build & test
 
 ```bash
@@ -147,12 +208,14 @@ make -f Makefile.gcc test
 `./run_e2e_test.sh` builds everything, launches the demo MARTe2 application
 (`Test/Configurations/streamhub_demo.cfg`: 3 UDPStreamers — multicast scalars,
 FirstSample/LastSample arrays, FullArray + uint64 ns time array) plus a
-StreamHub on port 8095, then runs the Go WS client `Test/E2E/streamhub` which
-verifies: `sources`/`config` events, ≥10 binary v1 pushes with wall-clock and
-strictly monotonic time on all streams, `stats` shape, a `zoom` round-trip
-(reqId echo, unicast), and a complete trigger cycle (setTrigger → arm →
-binary v2 capture → triggered → disarm). Logs land in
-`/tmp/streamhub_e2e_{marte,hub}.log`. Exit 0 iff every check passes.
+StreamHub on port 8095 (with history enabled in `/tmp/streamhub_e2e_history`),
+then runs the Go WS client `Test/E2E/streamhub` which verifies:
+`sources`/`config` events, ≥10 binary v1 pushes with wall-clock and strictly
+monotonic time on all streams, `stats` shape, a `zoom` round-trip (reqId echo,
+unicast), `historyInfo` broadcast (enabled, duration, decimation, signal count),
+a `historyZoom` round-trip (reqId echo, signal data), and a complete trigger
+cycle (setTrigger → arm → binary v2 capture → triggered → disarm). Logs land
+in `/tmp/streamhub_e2e_{marte,hub}.log`. Exit 0 iff every check passes.
 
 When changing the WS protocol, update **in lockstep**: this hub, the Go hub
 (`Common/Client/go/wshub`), the browser SPA (`Client/udpstreamer/static`), the