docs(cctv): describe the ground-height data plainly (#443)

Follow-up to #433.

src/data/local_data/cctv_ground_heights/README.md

@@ -1,37 +1,29 @@
 # CCTV ground heights
 
 Precomputed ground height under every camera in the served CCTV catalog and
-under the nine support points of its monitor plane, so the client can place a
-camera and its plane with no runtime sampling. Shared placement data: any
-consumer that renders these cameras on the same photorealistic surface can
-reuse it.
-
-Source surface: Google Photorealistic 3D Tiles, sampled at maximum detail with
-CesiumJS `scene.sampleHeightMostDetailed` from a running God's Eye View
-(scripts/precompute-cctv-heights.mjs). Heights are WGS84 **ellipsoidal**
-metres; no geoid conversion is needed or applied. This is placement data
-derived from the rendered surface, not imagery.
+under the nine support points of its monitor plane, aligned to work with
+Google Photorealistic 3D Tiles. Heights are WGS84 ellipsoidal metres. The
+client places a camera and its plane from this file with no runtime sampling.
 
 Runtime output: `cctv_ground_heights.json`
 
 ```
 {
   "schemaVersion": 1,
-  "provider": "google-photorealistic-3d",
+  "provider": "google-3d-tiles",
   "heightReference": "WGS84-ellipsoid",
   "generatedAt": "<ISO>",
   "cameras": {
     "<camera id>": {
-      "poseHash": "p1-…",          // hash of the served pose the samples describe
+      "poseHash": "p1-…",          // hash of the served pose the entry describes
       "status": "ok" | "miss",
       "mountGroundM": <number>,    // ground under the camera position (absent on a miss)
-      "supports": {                // ground under the plane's 3×3 support grid; a value is
-                                   // null where that point had no sample (absent on a miss)
-        "bl","bm","br",            // bottom row (left, middle, right)
-        "ml","mc","mr",            // middle row
-        "tl","tm","tr"             // top row
+      "supports": {                // ground under the plane's 3×3 support grid; null where
+        "bl","bm","br",            // a point has no value (absent on a miss)
+        "ml","mc","mr",
+        "tl","tm","tr"
       },
-      "misses": [<keys with no sample: "mount" and/or support keys>],
+      "misses": [<keys with no value>],
       "sampledAt": "<ISO>",
       "attempts": <n>
     }
@@ -40,19 +32,15 @@ Runtime output: `cctv_ground_heights.json`
 ```
 
 Support point positions are defined by `src/data/cctvFootprint.js`
-(`planeSupportPoints`), which the client, the server join
-(`server/providers/cctv/groundHeights.js`) and the precompute all share. An
-entry is used only while the camera's served pose still hashes to
-`poseHash`; a camera whose feed moved it, or whose pose a user edited, falls
-back to runtime placement from the Re:Earth DEM.
+(`planeSupportPoints`), shared with the server join
+(`server/providers/cctv/groundHeights.js`). An entry is used only while the
+camera's served pose still hashes to `poseHash`; a camera whose feed moved it,
+or whose pose a user edited, falls back to runtime placement from the terrain
+proxy.
 
-Regenerate (resumable; only cameras whose pose changed or whose entry is
-incomplete are re-sampled):
+Regenerate against a running app (resumable; only cameras whose pose changed
+or whose entry is incomplete are redone):
 
 ```
 GEV_BASE=http://localhost:4173 node scripts/precompute-cctv-heights.mjs
 ```
-
-Coverage at generation (2026-09-14): 3,445 of 3,446 cameras `ok` (six of them
-with one or more null supports), 34,444 sampled heights, 43 minutes on a real
-GPU. Regenerate whenever a pack changes its poses or a new pack lands.

src/data/local_data/cctv_ground_heights/cctv_ground_heights.json

@@ -1,6 +1,6 @@
 {
   "schemaVersion": 1,
-  "provider": "google-photorealistic-3d",
+  "provider": "google-3d-tiles",
   "heightReference": "WGS84-ellipsoid",
   "generatedAt": "2026-09-14T01:42:02.486Z",
   "cameras": {