build: pin file.encoding so the Maven build generates the parser on Windows

JJTree reads the .jjt under GRAMMAR_ENCODING but writes the intermediate .jj
with the JVM default charset. On a platform whose default is a legacy code
page the grammar's Unicode character classes are written lossily, and JavaCC
then rejects the mangled ranges:

    Error: Line 2331, Column 294: Right end of character range '?' has a
    lower ordinal value than the left end of character range '?'.

The Gradle build has pinned -Dfile.encoding=UTF-8 in gradle.properties since
2021, which is why only the Maven build is affected. Give it the same pin via
.mvn/jvm.config and put windows-latest back in the maven_verify matrix.

Author: knutwannheden

.github/workflows/ci.yml

@@ -64,8 +64,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        # windows-latest disabled: see #<issue> — record the reason, not just the comment
-        os: [ ubuntu-latest, macos-latest ]
+        os: [ ubuntu-latest, windows-latest, macos-latest ]
     steps:
       - uses: actions/checkout@v5
         with:

.mvn/jvm.config

@@ -0,0 +1 @@
+-Dfile.encoding=UTF-8

pom.xml

@@ -426,6 +426,11 @@
                         <goals>
                             <goal>jjtree-javacc</goal>
                         </goals>
+                        <!--
+                            GRAMMAR_ENCODING governs reading only: JJTree writes the intermediate
+                            .jj with the JVM default charset, which mangles the grammar's non-ASCII
+                            character classes unless file.encoding is UTF-8 (see .mvn/jvm.config).
+                        -->
                         <configuration>
                             <javaccCmdLineArgs>
                                 <arg>-CODE_GENERATOR:"Java"</arg>