docs: water at rest across the Core/Client/Server READMEs (terrain-water task 13)

Core: BlockRegistry gains WATER, with the reason it is wire-safe (block IDs
are never serialized) AND the reason it is a registry identity only rather
than a voxel the terrain path writes. Corrects the now-false line "Nothing
at runtime consumes the water data yet" -- it does, as of this task.
ChunkData's new per-column water fields and the Constants tunables.

Client: how the water sheet is built and, more importantly, WHY it is a
second mesh instead of a block in the density field -- the trap here is
that writing water into a Marching-Cubes field fuses it into the terrain
instead of laying it on top, and that is not obvious until you have done
it. Notes translucency came free from the separate material.

Server: the per-column water rule, which section is the authority for what
(WBID presence, WSRF level, WBTB fallback), that an unknown body leaves the
column dry rather than guessing, and why depth for shading comes from
blueprint heights rather than the clamped rendered geometry.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Server/README.md

@@ -27,6 +27,15 @@ vector ends up parallel to the view direction, so camera roll is undefined and G
 - **Road culling first.** Only the road segments whose bounding box reaches this chunk are kept,
   each tagged with its `RoadTier`. The padding is derived from the widest shoulder any tier has plus
   a margin, so widening a road cannot silently truncate it at chunk edges.
+- **Water per column** (task 13) — the blueprint is the **authority** on where water is; the server
+  only reads it. `WBID` decides presence (it is the water stage's own classification output, the
+  same set the biome grid's Ocean/Lake pixels form by construction), `WSRF` gives the surface level
+  per pixel, and the `WBTB` body table is the fallback when a column is flagged wet but carries the
+  `WSRF` no-water sentinel. A body the table does not know leaves the column **dry** rather than
+  guessing a level. The level is scaled by the same `HEIGHT_SCALE` as the terrain, so the sheet and
+  the seabed cannot drift apart. Depth for shading is taken from **blueprint** heights, not rendered
+  geometry — the terrain render clamps its floor at `Y = 2`, which would otherwise flatten every
+  deep-ocean column to one value. Each run logs what it drew (`[Server] Water at rest: …`).
 - **Surface height per column** (`GetExactSurface`) — the blueprint height scaled into the chunk's
   usable vertical band, then modified by any road carving.
 - **Density per voxel** — `(y − surfaceY)` normalised by the local slope, giving a signed distance to