Fix corrupted U+FFFD in biomodel_256364414.vcml test fixture

The Simulation Name (and matching Version Name) contained two U+FFFD
replacement characters where 'µ' (micron) used to be — a charset
corruption baked into the fixture: "Figure 2 is 4� micrometer radius".
The new TokenMangler validation correctly rejects U+FFFD as invalid
attribute content, breaking SEDMLExporterSBMLTest test case 91 on this
branch.

The pre-existing test only worked because TokenMangler.fixTokenStrict
silently mangled U+FFFD into '_' before SBML export. Replacing the bad
char with the actual 'µ' unicode letter doesn't help — fixTokenStrict
preserves Unicode letters, so the resulting SBML SId becomes invalid.
Substituting plain ASCII 'u' is round-trip-safe and self-explanatory:
"Figure 2 is 4u micrometer radius".

Verified via mvn -pl vcell-core -Dtest=SEDMLExporterSBMLTest test
restricted to this single fixture.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: jcschaff

vcell-core/src/test/resources/org/vcell/sbml/vcml_published/biomodel_256364414.vcml

@@ -2913,7 +2913,7 @@ cloned from 'Simulation0' owned by user temp</Annotation>
           <GroupAccess Type="1" />
         </Version>
       </MathDescription>
-      <Simulation Name="Vary Bleach Radius, FluorD=1, Figure 2 is 4� micrometer radius">
+      <Simulation Name="Vary Bleach Radius, FluorD=1, Figure 2 is 4u micrometer radius">
         <Annotation>cloned from 'Vary Bleach Radius, FluorD=1_1' owned by user temp
 cloned from 'Simulation0_1' owned by user temp
 cloned from 'Simulation0_1' owned by user temp
@@ -2936,7 +2936,7 @@ cloned from 'Simulation0' owned by user temp</Annotation>
         <MeshSpecification>
           <Size X="402" Y="402" Z="3" />
         </MeshSpecification>
-        <Version Name="Vary Bleach Radius, FluorD=1, Figure 2 is 4� micrometer radius" KeyValue="256364412" BranchId="256354044" Archived="0" Date="24-May-2023 20:41:20" FromVersionable="false">
+        <Version Name="Vary Bleach Radius, FluorD=1, Figure 2 is 4u micrometer radius" KeyValue="256364412" BranchId="256354044" Archived="0" Date="24-May-2023 20:41:20" FromVersionable="false">
           <Owner Name="les" Identifier="6" />
           <GroupAccess Type="1" />
           <Annotation>cloned from 'Vary Bleach Radius, FluorD=1_1' owned by user temp