Implemented history writer

Docs/StreamHub-API.md

@@ -101,6 +101,27 @@ Every transition is broadcast as a `triggerState` event.
 - `signals` — comma-separated full keys; absent → all signals of all sources.
 - The reply is **unicast** to the requesting client only.
 
+### `historyZoom`
+
+```json
+{"type":"historyZoom","reqId":42,"t0":1765360000.0,"t1":1765370000.0,
+ "n":2400,"signals":"s1:Sine,s2:Wave"}
+```
+- Reads from **disk-backed history** instead of the in-memory ring buffer.
+  Identical semantics to `zoom` but queries the `.shist` files written by
+  `HistoryWriter`.
+- `reqId`, `t0`/`t1`, `n`, `signals` — same meaning as `zoom`.
+- If history is not enabled, the reply contains `"error":"history not enabled"`.
+- Reply is **unicast** (same shape as `zoom` reply, but `"type":"historyZoom"`).
+
+### `historyInfo`
+
+```json
+{"type":"historyInfo"}
+```
+Request the hub to send a `historyInfo` event (unicast). Also sent automatically
+on client connect.
+
 ### `setMaxPoints`
 
 ```json
@@ -164,6 +185,34 @@ trigger has fired.
 `t` is serialised with `%.17g` (full float64 precision — required for
 µs windows at Unix-epoch magnitudes), `v` with `%.9g`.
 
+### `historyInfo`
+
+Sent on client connect (if history is enabled) and on `historyInfo` command:
+
+```json
+{"type":"historyInfo","enabled":true,"durationHours":1.0,"decimation":10,
+ "signals":{
+   "scalar:Sine1":{"t0":1765360000.0,"t1":1765370000.0,"count":360000,"capacity":360000},
+   "scalar:Sine2":{"t0":1765360000.0,"t1":1765370000.0,"count":360000,"capacity":360000}}}
+```
+- `enabled` — `true` if the `+History` config block is present and valid.
+- `durationHours` — configured history duration.
+- `decimation` — samples-to-disk decimation factor (1 = every sample).
+- `signals` — per-signal metadata keyed by `"sourceId:signalName"`:
+  - `t0`/`t1` — oldest/newest timestamp stored on disk (Unix seconds).
+  - `count` — number of valid entries currently in the circular file.
+  - `capacity` — total capacity of the circular file.
+
+### `historyZoom` (reply)
+
+Same shape as `zoom` reply, but `"type":"historyZoom"`:
+
+```json
+{"type":"historyZoom","reqId":42,"signals":{
+  "s1:Sine":{"t":[1765360000.1230000,"…"],"v":[0.123456789,"…"]}}}
+```
+If history is not enabled: `{"type":"historyZoom","error":"history not enabled"}`.
+
 ### `maxPointsUpdated`
 
 ```json