test: review-driven determinism + coverage (#953 #963)

From the Hoshe/Tyre review:
- drainage: assert flow_accumulation/max_accumulation determinism + clamp ≥ 1
  (the D-209 strength denominator); isolated-basin merge path (no panic).
- subbiome: each derivable variant reachable + Volcanic never emitted.
- planet_simulation: new test_sim_determinism.py — same body simulates to a
  bit-identical elevation array at 1024×512 (the 271-body bake can't be cheaply
  re-run, so a silent drift = full re-bake). Verified PASS.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tooling/planet-gen/test_sim_determinism.py

@@ -0,0 +1,71 @@
+#!/usr/bin/env python3
+"""Determinism guard for planet_simulation.simulate() (#963).
+
+The 1024×512 elevation bump (D-202 amended) claims to be deterministic — the
+271-body heightmap bake cannot be re-run cheaply, so a silent non-determinism
+means a full regeneration. This asserts that simulating the same body twice
+yields a bit-identical elevation array.
+
+Run:  uv run python tooling/planet-gen/test_sim_determinism.py
+Exit: 0 = deterministic, 1 = drift detected, 2 = could not find a test body.
+"""
+import hashlib
+import sqlite3
+import sys
+from pathlib import Path
+
+REPO = Path(__file__).resolve().parent.parent.parent
+sys.path.insert(0, str(REPO / "tooling" / "planet-gen"))
+
+import numpy as np  # noqa: E402
+
+from body_definition_parser import parse_system  # noqa: E402
+from planet_simulation import GRID_H, GRID_W, simulate  # noqa: E402
+
+
+def _elev_hash(terrain) -> str:
+    arr = np.ascontiguousarray(terrain["elevation"], dtype=np.float32)
+    return hashlib.sha256(arr.tobytes()).hexdigest()
+
+
+def main() -> int:
+    conn = sqlite3.connect(str(REPO / "server" / "data" / "systems.db"))
+    rows = conn.execute(
+        "SELECT body_id, terrain_reference FROM bodies "
+        "WHERE inhabited=1 AND terrain_reference IS NOT NULL AND population>0 "
+        "AND system_id != 'GJ 0' ORDER BY body_id LIMIT 30"
+    ).fetchall()
+
+    checked = 0
+    for body_id, tref in rows:
+        if checked >= 3:
+            break
+        sys_index = tref.split("/bodies/")[0] + "/index.md"
+        try:
+            defs = parse_system(sys_index)
+        except Exception:
+            continue
+        bd = next((d for d in defs if d.get("id") == body_id), None)
+        if bd is None:
+            continue
+        t1 = simulate(bd)
+        t2 = simulate(bd)
+        if not t1 or "elevation" not in t1:
+            continue
+        h1, h2 = _elev_hash(t1), _elev_hash(t2)
+        ok = h1 == h2 and t1["elevation"].shape == (GRID_H, GRID_W)
+        print(f"  {body_id}: {'OK' if ok else 'DRIFT'}  {h1[:16]}  shape={t1['elevation'].shape}")
+        if not ok:
+            print(f"FAIL: {body_id} elevation is non-deterministic ({h1[:16]} != {h2[:16]})")
+            return 1
+        checked += 1
+
+    if checked == 0:
+        print("SKIP: no testable body found")
+        return 2
+    print(f"PASS: {checked} bodies simulate deterministically at {GRID_W}×{GRID_H}")
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())