raster auto-center: make vertical line-scan Y padding configurable

The 2nd-stage vertical line scan (n_x=1, height from the ML box) padded its
height by a hardcoded 0.10 each side (20% total), which was often too short to
cover the whole loop. Read the per-side fraction from
daq.auto_raster.line_scan_y_padding_fraction (default 0.15) so it can be tuned
per beamline.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/aare/common/config/x06da.yaml

@@ -56,3 +56,4 @@ daq:
   auto_raster:
     loop_face_padding_fraction: 0.15   # pad loop_face raster boxes by 15% per side
     include_crystal: false             # extend the grid to cover crystals outside the loop box
+    line_scan_y_padding_fraction: 0.15 # pad the 2nd-stage vertical line scan height by this per side (10-20%)

src/aare/daq/operations/raster/service.py

@@ -213,7 +213,7 @@ class RasterService:
         grid_size_mm: Coordinate,
         default_n_y: int = 50,
         y_retarget_threshold_mm: float | None = None,
-        y_padding_fraction_each_side: float = 0.10,
+        y_padding_fraction_each_side: float = 0.15,
     ) -> tuple[SmargonCoordinate, int]:
         geom = self.ctx.sample_geometry
         beam_x_pxl = geom.beam_location_pxl.x
@@ -799,7 +799,9 @@ class RasterService:
                 geom.beam_size_mm.y * 2.0,
                 grid.grid_size_mm.y * 4.0,
             ),
-            y_padding_fraction_each_side=0.10,
+            y_padding_fraction_each_side=float(
+                cfg_get("daq.auto_raster.line_scan_y_padding_fraction", 0.15)
+            ),
         )
 
         self.logger.info(