Valley shaping fixes from the first transects

- Head ring now covers every cell that can sit beside water
- Blend widths are smoothed along the course; the bed is a broad bowl
- Diagonal shore cells keep the lip; cross-section rows belong to their own station
- Transect probe prints coordinates and numeric section tables

probe/src/test/java/art/arcane/iris/probe/RiverTransectProbeTest.java

@@ -42,6 +42,10 @@ public class RiverTransectProbeTest {
         assertEquals(1, summary.oceanWrites());
         assertEquals(1, summary.uncontainedWetCells());
         assertFalse(summary.passes());
+        assertEquals(3, summary.details().size());
+        assertTrue(summary.details().get(0).startsWith("uncontained 0,0"));
+        assertTrue(summary.details().get(1).startsWith("oceanWrite 5,5"));
+        assertTrue(summary.details().get(2).startsWith("bankStep 1"));
     }
 
     @Test