Add continuity-win fixture and room-router tests

Adds narrow_channel.dsn: a keepout wall with a narrow gap positioned off the
routing grid, so no grid cell centre falls inside the gap's free centreline.
The exact/grid track drops the net (cell quantization can't fit a centreline
in the channel); the continuous room track routes it. This is the headline
result that continuous free-space beats the grid.

Tests: the room router's output is DRC-clean (exact has_violation is None) on
simple/crossing/kicad/narrow_channel; it connects simple/crossing (with a via)
and all four multi-pin KiCad nets; traces are orthogonal and end on pads; and
the continuity contrast asserts exact routes 0 while rooms routes 1, both
clean. Oracle-gated connectivity parity on simple/crossing.

tests/dsn/fixtures/narrow_channel.dsn

@@ -0,0 +1,65 @@
+(pcb "narrow_channel.dsn"
+  (parser
+    (string_quote ")
+    (space_in_quoted_tokens on)
+    (host_cad "freeroute-test")
+    (host_version "1.0")
+  )
+  (resolution um 10)
+  (unit um)
+  (structure
+    (layer F.Cu
+      (type signal)
+      (property
+        (index 0)
+      )
+    )
+    (layer B.Cu
+      (type power)
+      (property
+        (index 1)
+      )
+    )
+    (boundary
+      (path pcb 0  0 0  200000 0  200000 -30000  0 -30000  0 0)
+    )
+    (keepout "wall_left"
+      (rect F.Cu 0 -14000 94000 -16000)
+    )
+    (keepout "wall_right"
+      (rect F.Cu 102000 -14000 200000 -16000)
+    )
+    (rule
+      (width 2000)
+      (clearance 2000)
+    )
+  )
+  (library
+    (padstack Rect_Pad
+      (shape (rect F.Cu -1000 -1000 1000 1000))
+      (attach off)
+    )
+    (image PAD
+      (pin Rect_Pad 1 0 0)
+    )
+  )
+  (placement
+    (component PAD
+      (place C1 98000 -5000 front 0)
+      (place C2 98000 -25000 front 0)
+    )
+  )
+  (network
+    (net NET_C
+      (pins C1-1 C2-1)
+    )
+    (class default
+      (rule
+        (width 2000)
+        (clearance 2000)
+      )
+    )
+  )
+  (wiring
+  )
+)