W3: the Cove (bay, pond, pier for sea fishing), mackerel and cod (cod only in deep water), non-start worldlets offered for testing when run from source

docs/WORLDLETS_DESIGN.md

@@ -1,9 +1,33 @@
 # Worldlets design
 
-Status: **W0, W1 and W2 built**; the rest is planned. The journey between
+Status: **W0-W3 built** (the Cove, without its stream yet); the rest is
+planned. The journey between
 worldlets is §10, and the two ways to play (the journey, or one land) are in
 its decisions.
 
+*As built in W3:*
+- `data/worldlets/cove.json`: low hills, mountains behind, a wide sandy
+  bay (a `beach` block) and a small pond. Not a start; its `next` is back
+  to the Valley (tunnel, J1) and on to the Highlands (sail).
+- The pier (`scripts/world/pier.gd`), a world "place" (`"places":
+  [{"type": "pier"}]`): planks on posts from where the sand is at deck
+  height to 24 m past the waterline. Worked out from the terrain, so it's
+  the same for everyone and never saved or dismantled; scenery keeps clear
+  of it. ~1.9 m of sea at its end, ~2.7 m a cast beyond.
+- Sea fish in `data/fish.json`: mackerel (common) and cod, which only bites
+  in water 2.2 m or deeper (a per-fish `min_depth`). Grilled at a campfire,
+  salted, or smoked, like the lake's.
+- Running from the source, the New World and Host pages also offer
+  worldlets that aren't starts, marked "(testing)", so the Cove can be
+  played before the tunnel exists. Release builds offer only the Valley.
+- No sea stacks without a sea cliff.
+- 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).
+
 *As built in W2:*
 - `lake` blocks can repeat: every lake and pond is its own body
   (`terrain.lakes`, `world.water_bodies()`), all at one surface