Show more rivers (riverThreshold 50->25) + remove globe subgrid overlay

Two fixes from user feedback on the hydrology view:

1. Rivers looked too sparse. Investigated coupling rainfall into discharge
   (climate precip -> river rainfall): it does NOT work as hoped because river
   *location* is set by drainage topology (where water collects), not local
   rainfall -- weighting rainfall mostly changes river *size*, and concentrating
   a fixed water budget pushes mid-size rivers below the display threshold, so
   FEWER rivers show. Reverted that approach. Instead lower the default
   riverThreshold 50 -> 25 so tributaries render too: a richer, more visible
   network (~2x river cells on test worlds) without descending into noise.
   Tunable in planet.cfg.

2. Clicking a tile overlaid a low-res, always-elevation-coloured subgrid patch
   on the 3D globe that clashed with the active colour mode and read as a
   "strange pattern". Removed the globe overlay; the clicked tile's high-res
   subgrid still shows in the right-side detail panel.

test_logic + test_biota pass; full app builds clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

BUILD.md

@@ -113,7 +113,7 @@ Drift keeps running at a finer timestep during hydrology. (Config keys keep the
     phase3AfterMy      300 My drift before the "Start Phase 2.5?" prompt
     phase3DtScale      0.2    hydrology timestep = cflDtMy()*this (finer = slower drift)
     rainfall           1.0    uniform precip per cell (drainage-area unit)
-    riverThreshold     50     discharge above which a cell is a river
+    riverThreshold     25     discharge above which a cell is a river (lower = richer network)
     riverIncision      0.02   K in stream-power incision K*Q^m*S^n*dt (raise = carve faster)
     riverDischargeExp  0.5    m (discharge exponent)
     riverSlopeExp      1.0    n (slope exponent)

CLAUDE.md

@@ -563,7 +563,7 @@ triangles (plates are fixed in phase 1).
   before the Phase-3 prompt; `phase3DtScale` (0.2) — Phase-3 timestep =
   `cflDtMy()*this` (smaller = finer carving, slower drift per step); `rainfall`
   (1.0) — uniform precip per cell (drainage-area unit; orographic precip is a later
-  climate add-on); `riverThreshold` (50) — discharge above which a cell is a river
+  climate add-on); `riverThreshold` (25) — discharge above which a cell is a river
   (also the river-render threshold); `riverIncision` K (0.02) + `riverDischargeExp`
   m (0.5) + `riverSlopeExp` n (1.0) — stream-power incision `K*Q^m*S^n*dt` (raise K
   for faster valley carving); `riverTransport` (0.1) — transport capacity

src/render/ViewerRender.cpp

@@ -34,8 +34,9 @@ void Viewer::renderGlobe3D() {
     }
     rlEnd();
 
-    if (selectedCell >= 0)                                    // detail patch
+    // (The clicked tile's high-res subgrid is shown in the right-side detail panel,
-        drawSubgrids(subgrids, visBase, elevExagg, planet.cfg.seaLevel, 0.0016f);
+    // not overlaid on the globe -- the overlay was a low-res, always-elevation-coloured
+    // patch that clashed with the active colour mode and read as a "strange pattern".)
 
     if (showBorders && (!borders.empty() || !ridgeBorders.empty())) {
         rlSetLineWidth(2.0f); rlBegin(RL_LINES);

src/sim/PlanetTypes.hpp

@@ -158,7 +158,8 @@ struct PlanetConfig {
     double phase3AfterMy     = 300.0; // My of drift before the Phase-3 prompt
     double phase3DtScale     = 0.2;   // Phase-3 timestep = cflDtMy() * this (finer)
     double rainfall          = 1.0;   // uniform precip per cell (drainage-area unit)
-    double riverThreshold    = 50.0;  // discharge above which a cell counts as a river
+    double riverThreshold    = 25.0;  // discharge above which a cell counts as a river
+                                       // (lower = richer network incl. tributaries shown)
     double riverIncision     = 0.02;  // K in stream-power incision K*Q^m*S^n*dt
     double riverDischargeExp = 0.5;   // m: discharge exponent in stream power
     double riverSlopeExp      = 1.0;  // n: slope exponent in stream power