fix(simulation): PR #210 review round — guard boundary, live oasis pinning, unreachability proof (T-964)

Guard becomes land_districts <= 1 (both reviewers converged — a lone
island definitionally cannot show two distinct directions; same
nothing-to-vary condition one value short), with a lone-island vacuous-
pass fixture; golden confirmed untouched. Oasis scaling adjudicated as
LIVE, not future — GRID_W is already 1024 on main, so ring iterations
change 2/4 -> 4/8 today: extracted a pure oasis_ring_iterations()
helper pinned by tests at both 512 and 1024, and traced exactly why the
determinism hash stayed green (it reads only elevation; the rings touch
only biome — a genuinely different array, not a coincidence). The
drainage merge-logic question answered byte-precisely: zero logic
changed vs main (comment-only diff) — and the deeper dig PROVED the
'isolated basin with another basin to escape to' branch is
mathematically unreachable for any connected grid (contracting vertex
groups of a connected graph cannot disconnect it), so the comment now
states that instead of narrating a divergence that never fires; two
direct merge-target tests added regardless. Wrap test renamed to what
it actually pins (non-wrap-awareness). D-010 docstring softened to
same-process purity, naming the cascade golden as the cross-run layer.

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

Makefile

@@ -259,6 +259,9 @@ test-tooling:
 	elif [ $$rc -ne 0 ]; then \
 		echo "  FAIL: planet_simulation determinism drift (exit $$rc)"; exit $$rc; \
 	fi
+	@echo "  [test-tooling] oasis ring-scaling pin (T-964, PR #210 review)..."
+	@$(VENV_PY) tooling/planet-gen/test_oasis_ring_scaling.py 2> .cache/test-tooling-oasis-ring.log || \
+		{ echo "  FAIL: oasis ring scaling — log follows:"; cat .cache/test-tooling-oasis-ring.log; exit 1; }
 	@echo "  [test-tooling] economy_import.traits validation units (T-995/PR #173 H2)..."
 	@mkdir -p .cache
 	@python3 tooling/economy-db/test_traits.py 2> .cache/test-tooling-traits.log || \