fix(build): pin icu >=75,<76 to prevent header/lib version mismatch

The build environment resolved ICU 78 headers while the host environment
had ICU 75 libraries (constrained by xerces-c, qt6-main, etc.). This
caused libFreeCADBase.so to contain icu_78 mangled symbols that couldn't
be found in the bundled ICU 75 libs at runtime:

  undefined symbol: _ZN6icu_7813UnicodeString8fromUTF8ENS_11StringPieceE

Pin ICU explicitly in both recipe.yaml host deps and pixi.toml to ensure
headers and libs always match.

package/rattler-build/recipe.yaml

@@ -102,6 +102,7 @@ requirements:
     - fmt
     - freetype
     - hdf5
+    - icu>=75,<76
     - lark
     - libboost-devel
     - matplotlib-base

pixi.toml

@@ -25,6 +25,7 @@ freetype = "*"
 git = "*"
 graphviz = "*"
 hdf5 = "*"
+icu = ">=75,<76"
 ifcopenshell = "*"
 lark = "*"
 libboost-devel = "*"