[Icons] Keep the System semantic ladder on the text floor

System authors no palette of its own, so its semantic family is fixed data
measured against whatever surfaces the platform provides. The ladder that
landed with the authored roles reused the old contrast-ordered values:
PositiveSoft and NegativeSoft sat at 2.76–3.90:1 on common light and dark
platform surfaces, and the strong level was less saturated than the
secondary, which is the inversion the chroma rule removes.

Re-author the six default semantic colors per scheme at the same 4.60
target the shipped themes use, against representative platform surfaces,
and mirror them in System's palette files. The new
DefaultSemanticFamilyMeetsTheLadderOnPlatformSurfaces test holds the floor
and the saturation order there, and shippedPalette's early return now says
why System is outside the authored-palette assertions.

cockatrice/themes/System/palette-default-dark.toml

@@ -9,8 +9,8 @@
 AccentStrong         = #ff148c3c
 AccentSoft           = #ff78c850
 Positive             = #ff6dd58c
-PositiveStrong       = #ff99e2ae
-PositiveSoft         = #ff5e8873
+PositiveStrong       = #ff32c05c
+PositiveSoft         = #ff8ab196
 Negative             = #ffff8a80
-NegativeStrong       = #ffffb3ac
-NegativeSoft         = #ff9b7678
+NegativeStrong       = #ffff8378
+NegativeSoft         = #ffc79e9b

cockatrice/themes/System/palette-default-light.toml

@@ -8,9 +8,9 @@
 [AppColors]
 AccentStrong         = #ff148c3c
 AccentSoft           = #ff78c850
-Positive             = #ff1b7f3b
-PositiveStrong       = #ff104a22
-PositiveSoft         = #ff5d7f6f
+Positive             = #ff1a7d3a
+PositiveStrong       = #ff007e28
+PositiveSoft         = #ff4c7659
 Negative             = #ffb3261e
-NegativeStrong       = #ff7c1b15
-NegativeSoft         = #ff9a6c6f
+NegativeStrong       = #ffda0b00
+NegativeSoft         = #ff935f5c