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.
Ryan Malloy
faf2df42bc