The Cove's stream: a stream block whose water only runs downhill (spring to the bay), its own water ribbon, trout, on the map; scenery and landmarks keep clear

docs/WORLDLETS_DESIGN.md

@@ -52,9 +52,14 @@ its decisions.
 - Tests: `test_cove` (walk onto the pier, catch a mackerel off its end, cod
   only in deep water), and the Cove in `test_terrain_edges` and
   `test_boundary`.
-- Still to do: the stream (it needs water that runs downhill, which the
-  one flat water surface can't do: its own ribbon of water, planned with
-  the Cove's sea cave when the tunnel arrives).
+- The stream (after 0.4.0): a `stream` block, a line of points from a
+  spring under the mountains to the bay. Its water surface is the running
+  minimum of the ground along it (so it only ever runs downhill, down to
+  sea level), the bed is cut 0.6 m under that and the banks rise a little
+  above it; its water is its own ribbon mesh (`terrain._stream_mesh()`),
+  murkier than the lake. `water_at()` answers "stream" (trout bite there,
+  `data/fish.json`), the map shows it, and scenery, the den, the barrow and
+  the stones keep clear of it (`terrain.near_stream()`).
 
 *As built in W2:*
 - `lake` blocks can repeat: every lake and pond is its own body