Store the depth channel in an analysis result and serve it to the 2D depth view

A replay could draw arrows but not depth, and the recording view said there was
nothing to draw even when a result existed. Depth is now quantised per frame
(14 B empty, 486 B at the worst frame seen), read from the cache by the same
draw path live uses, and the raw-recording note appears only when no result
exists. Format version 2; see apps/studio/docs/replay-analysis.md.

apps/studio/config/locales/en_US.json

@@ -190,7 +190,7 @@
     "view_mode_tip": "Image display mode while running",
     "replay_pipeline": "Recorded video (raw camera image)",
     "replay_pipeline_tip": "Replay shows the recorded raw camera image with no live arrow overlay; to check the algorithm, use View → Recompute forces from the video",
-    "replay_raw_note": "A recording stores only the raw camera image: there is no flow or depth to draw. Use “Recompute from the video” for the algorithm's output",
+    "replay_raw_note": "A recording stores only the raw camera image: there is no flow or depth to draw, because this recording has no analysis result yet. Opening a recording runs the pass automatically, or press “Recompute from the video”; once a result exists this pane can draw arrows and the depth map",
     "frame_missing_note": "This frame carries no {fields}: there is nothing for this view to draw",
     "field_flow": "flow",
     "field_depth": "depth",
@@ -201,6 +201,7 @@
     "arrow_empty_tip": "The deformation arrow image appears here while running or previewing",
     "depth_unavailable": "No depth data for this frame\n(not a depth of zero, and no silent fallback to the processed image)",
     "depth_scale": "Depth scale {low} – {high}",
+    "depth_empty": "This frame's depth is all zeros (0 of {pixels} px non-zero): the ramp's low colour is a real zero response, not missing data",
     "arrow_scale": "Arrows: displacement ×{factor}",
     "arrow_scale_tip": "Display magnification of the relative displacement: the arrows are drawn from the algorithm output of the captured image, with the measured displacement multiplied by this factor and capped at 0.8 of the grid spacing (a capped arrow no longer shows its true factor).",
     "arrow_pipeline": "Captured image → algorithm output",