docs: water sections (WBID/WBTB/WSRF), no-BSIN rationale, SkipRoads + water stage in READMEs (terrain-water task 03)

Byte-accurate entries for the three water sections (transitional
per-body level rule, provisional salinity, WSRF sentinel/quantization),
updated size math (~576 MiB at 8K with water), the on-the-record
rationale for NOT serializing basins, and README updates for the new
pipeline stage, the 0_water snapshot, and the SkipRoads toggle.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Tools/README.md

@@ -17,6 +17,10 @@ Builds the topographical and logistical foundation of the island before any 3D e
 - **Topography:** FastNoiseLite plus distance-falloff maths for a guaranteed mainland island, a
   mountain spine, dynamic temperature-driven sea level, and an impact crater pushed into the northern
   coast (carved at 80 % of its radius so a landbridge always survives).
+- **Water bodies:** the classified water promoted into explicit blueprint data (ocean + labeled
+  lakes with transitional per-body levels — `WBID`/`WBTB`/`WSRF` sections), plus priority-flood
+  basin diagnostics. The `SkipRoads` config toggle turns a full ~26-min generation into a ~80 s
+  road-less iteration cycle.
 - **Logistics:** `AStarGrid2D` pathfinding producing a looping continental highway, a branch to the
   mountain hub, and county roads daisy-chained outward with Prim's algorithm. Paths are decimated
   (Ramer–Douglas–Peucker) then smoothed (4 Chaikin passes).