controller: snapshot history + layout_map + UI grid

snapshot_history.py — async periodic capture per instance:
  interval_sec / keep_last (FIFO eviction) / dir configurable
  GET /snapshots/{instance}?limit=N → list metadata
  GET /snapshots/{instance}/{filename} → image bytes
  Persisted в /var/lib/cuda-grid/snapshots/{instance}/<ts>.png

layout_map / layout_filter_target в InstanceCfg — для будущей runtime switch
архитектуры (через streamselect либо filter rework — выбор за Phase 7).
Текущий _set_layout dispatches к layout_filter_target c index из map.

UI:
  + Layout buttons (quad/single/main_plus_preview placeholder)
  + Snapshot history grid с thumbnails (loaded /snapshots/{inst}?limit=24)
  + "Reload history" button

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

controller/cuda_grid_controller/dispatch.py

@@ -219,12 +219,12 @@ class CommandDispatcher:
     # ─── Layout ────────────────────────────────────────────────────
 
     async def _set_layout(self, inst: InstanceCfg, layout: str) -> None:
-        if layout not in PREDEFINED_LAYOUTS:
+        if layout not in inst.layout_map:
             log.warning(
                 "dispatch.unknown_layout",
                 instance=inst.name,
                 layout=layout,
-                available=PREDEFINED_LAYOUTS,
+                available=list(inst.layout_map.keys()),
             )
             return
 
@@ -232,12 +232,13 @@ class CommandDispatcher:
         client = self._client(inst)
         try:
             reply = await client.send_command(
-                inst.filter_target, "layout", layout
+                inst.layout_filter_target, "map", str(inst.layout_map[layout])
             )
             log.info(
                 "dispatch.layout_set",
                 instance=inst.name,
                 layout=layout,
+                index=inst.layout_map[layout],
                 ffmpeg_reply=reply,
             )
         except (TimeoutError, Exception) as e: