fix(skewt): clip grid + traces to the plot box

Skewed isotherms, dry adiabats, and mixing-ratio lines were drifting
outside the frame at the corners — most visibly the warm-side
isotherms exiting the right edge near the top of the diagram. The
existing `clip_polyline/1` only filtered with a ±200 px slack, so
segments crossing the boundary still drew past the frame.

Wrap every grid layer + the T/Td/parcel polylines in a
`<g clip-path="url(#skewt-plot-clip)">`. The clipPath defines a
rectangle from (left, top) to (right, bottom). Frame outline and
axis labels stay outside the clip group so they keep rendering at
the box edge unchanged. The hover-cursor crosshair also lives
outside the clip so its readout box can sit flush against the plot
corner without being trimmed.

mix test: 2,902 / 2,902 + 221 properties green. mix credo --strict
clean.

lib/microwaveprop_web/live/skewt_svg.ex

@@ -43,18 +43,29 @@ defmodule MicrowavepropWeb.SkewtSvg do
     cleaned = clean_profile(profile)
     parcel = Keyword.get(opts, :parcel_trace, [])
 
+    plot_w = @right - @left
+    plot_h = @bottom - @top
+
     [
       ~s|<svg viewBox="0 0 #{@width} #{@height}" xmlns="http://www.w3.org/2000/svg" class="w-full h-auto" role="img" aria-label="Skew-T-Log-P diagram">|,
       style_block(),
+      # Define a rectangular clip the plot interior shares so any
+      # grid/trace polyline whose vertices stretch past the frame
+      # (e.g. skewed isotherms exiting the right edge near the top)
+      # is visually cropped. The frame and axis labels live *outside*
+      # this group, so the box outline still reads clean.
+      ~s|<clipPath id="skewt-plot-clip"><rect x="#{@left}" y="#{@top}" width="#{plot_w}" height="#{plot_h}"/></clipPath>|,
+      ~s|<g clip-path="url(#skewt-plot-clip)">|,
       isobars(),
       isotherms(),
       dry_adiabats(),
       moist_adiabats(),
       mixing_ratio_lines(),
-      frame(),
-      axis_labels(),
       profile_traces(cleaned),
       parcel_trace_line(parcel),
+      "</g>",
+      frame(),
+      axis_labels(),
       cursor_layer(),
       "</svg>"
     ]