feat(vision): add screenshot diff and stability helpers

docs/API.md

@@ -282,6 +282,70 @@ Notes:
 - If `wait_for_window=true`, the server polls for a matching window and returns `window_found`.
 - `dry_run=true` returns the resolved argv/cwd without launching.
 
+## `POST /vision/diff`
+
+Measure whether a screen region changed meaningfully between two captures.
+
+Query params:
+
+- `screen` (int, default `0`) - used for `mode=screen` and `mode=region`
+
+Compare live captures:
+
+```json
+{
+  "mode": "region",
+  "region_x": 120,
+  "region_y": 80,
+  "region_width": 600,
+  "region_height": 300,
+  "delay_ms": 400,
+  "diff_threshold": 0.01
+}
+```
+
+Compare provided images:
+
+```json
+{
+  "mode": "image",
+  "before_image_base64": "iVBORw0KGgoAAA...",
+  "after_image_base64": "iVBORw0KGgoBBB...",
+  "diff_threshold": 0.01
+}
+```
+
+Response includes:
+- `diff_ratio` — average normalized pixel difference
+- `changed` — whether `diff_ratio >= diff_threshold`
+- `region` — compared region
+
+## `POST /vision/stability`
+
+Measure whether a screen region stays visually stable over a short interval.
+
+Query params:
+
+- `screen` (int, default `0`)
+
+```json
+{
+  "region_x": 0,
+  "region_y": 0,
+  "region_width": 1920,
+  "region_height": 1080,
+  "sample_interval_ms": 250,
+  "duration_ms": 1200,
+  "diff_threshold": 0.005
+}
+```
+
+Response includes:
+- `stable`
+- `sample_count`
+- `max_diff_ratio`
+- `avg_diff_ratio`
+
 ## `POST /wait`
 
 Wait on a structured UI condition instead of guessing sleep durations.