W0: terrain from worldlet building blocks (Valley in data/worldlets, identical heights), WorldInfo API, worldlet in saves and servers; the cliff tunnel plan

docs/WORLDLETS_DESIGN.md

@@ -1,7 +1,23 @@
 # Worldlets design
 
-Status: **proposal**. Nothing here is built yet, apart from the first piece
-of groundwork (`world.water_at()` and `data/fish.json`, see §6).
+Status: **W0 built**; the rest is planned. The journey between worldlets is
+§10.
+
+*As built in W0:*
+- `data/worldlets/valley.json`: the Valley as data, its terrain as five
+  building blocks in order (`hills`, `rim`, `sea_cliff`, `lake`,
+  `ocean_edge`) with every number that used to be hard-coded.
+- `terrain.gd` runs a worldlet's blocks (`use_worldlet()`); each block is
+  the old code's arithmetic, unchanged. `tests/test_worldlets.gd` checks
+  four seeds give exactly the same height grid as before (SHA-256 of the
+  grid, taken from the old code).
+- The worldlet is chosen like the seed: a server's (it sends its real one),
+  else the save's `meta.json` "worldlet", else the Valley (worlds from
+  before W0). New worlds and servers record it.
+- The WorldInfo API on the world: `worldlet_id()`, `water_bodies()`,
+  `climate()`, `crossings()` (the planned ways on), beside `water_at()`.
+  Systems still read the terrain's numbers directly; they move over to
+  these as each new worldlet needs them to differ (W1-W3).
 
 ## 1. What a worldlet is
 A **worldlet** is a small, complete world type: a ~320 m playable area with
@@ -224,6 +240,41 @@ you've learned and gathered there:
 - **Through:** the far end opens onto the next worldlet. Walking out loads
   it (a short fade, "The Cove"), with your bag, cloak and stats as they are.
 
+### The first tunnel: down the sea cliff to the Cove
+The Valley's sea side ends at a sheer cliff, its top about level with the
+valley and the sea some 25 m below. The Cove is down there, so the first
+tunnel goes **down**, inside the cliff, from the clifftop to the shore.
+- **The seam:** a cracked outcrop on the clifftop strip near the edge (the
+  sea_cliff block marks where), with old pick marks and loose stones at its
+  foot. It can't be dug until you have the antler pickaxe (tier 3); a lesser
+  pick just chips it ("Too hard: you need a better pick").
+- **Digging down:** the tunnel is about six stretches, each a few metres on
+  and a few down: a sloping passage that turns back on itself like a
+  stair, so the 25 m drop fits inside the cliff. A stretch takes many pick
+  blows (stamina, and wear on the pick) to open.
+- **Props:** each new stretch needs a **timber prop** (2 wood) set at its
+  mouth before you can dig the next. Leave one out and the face won't take
+  another blow ("It's loose: prop it first"); nothing caves in on you, so
+  it's a cost, not a trap. That's about 12 wood all the way down.
+- **Light:** the tunnel has no daylight at all. Every stretch is pitch dark
+  until you bring light: a torch in your hand works, but your hands are full
+  with the pick, so the way to do it is to set **candles and torch stands**
+  as you go (tallow candles from bear fat, torch stands from wood). Placed
+  lights stay, so the way is lit for the trip back and for friends.
+- **The bottom:** the last stretch breaks out into a sea cave, its mouth
+  opening on the Cove's shingle beach. Walking out of the mouth is the
+  crossing: a fade, then the Cove, arriving at the cave's Cove end. The way
+  back starts there and climbs the same tunnel.
+- **What's saved:** the tunnel belongs to the Valley's save: how far it's
+  dug, the props, the lights. On a server it's shared: anyone can dig on
+  from where someone stopped, and the lights are everyone's.
+- **How it's built:** the tunnel is its own small space (a "between" scene
+  next to the worldlet), made from a kit of rock pieces (straight, turn,
+  slope down, the cave mouth) laid out from the seed, with box-shaped
+  colliders. The heightmap can't have holes or overhangs, so the passage is
+  never cut into the terrain itself: you walk into the seam and you're in
+  the tunnel scene.
+
 ### Why go back
 - **Each worldlet has things the others lack.** Clay and salt only in the
   Valley and Cove; little wood in the Highlands; no bears outside the