fix: avoid Java 25 for legacy Fabric

Plain Craft Launcher 2/Modules/Minecraft/ModJava.vb

@@ -251,6 +251,11 @@ Public Module ModJava
                 '1.18+：Java 17+
                 AddConstraint(ValueRange(Of Version).AtLeast(New Version(17, 0)))
             End If
+            If Instance.Version.Fabric <> "未知版本" AndAlso
+               CompareVersion(Instance.Version.Fabric, "0.17.0") < 0 Then
+                'Fabric Loader 0.16.x 及更早版本的 Mixin/ASM 不兼容 Java 25
+                AddConstraint(ValueRange(Of Version).LessThan(New Version(25, 0)))
+            End If
         End If
 
         'LiteLoader 检测

docs/design/issue-9239-java-25-fabric-compatibility.md

@@ -0,0 +1,48 @@
+# Issue 9239: Java 25 and legacy Fabric compatibility
+
+## Context
+
+Issue #9239 reports that automatic Java selection fails for a Minecraft 1.21.4
+instance, while manually selecting Java 21 succeeds. The supplied PCL log shows
+that PCL selects Java 25 from the official runtime directory because it is first
+in the configured Java list and satisfies the current `[21.0, +infinity)`
+requirement.
+
+Although the issue description says no mods are installed, the supplied launch
+script and crash log show Fabric Loader `0.16.12`. The crash is caused by its
+Mixin/ASM stack attempting to read a Java 25 class (class-file major version
+69), which it does not support.
+
+## Current implementation
+
+- `GetJavaRequirement` in `Plain Craft Launcher 2/Modules/Minecraft/ModJava.vb`
+  assigns Minecraft 1.20.5+ a lower Java bound of 21.
+- Fabric instances only add lower bounds today, so Fabric 0.16.12 retains an
+  unbounded upper range.
+- Automatic selection walks the user-configured Java list and selects the first
+  checked Java in that range. The list sorter prioritizes official runtime
+  locations, making Java 25 selected before installed Java 21 entries.
+
+## Design
+
+For Fabric Loader versions older than `0.17.0`, add an exclusive Java 25 upper
+bound (`< 25.0`) during requirement analysis. This makes Java 21--24 eligible
+for Minecraft 1.21.4 while preventing the known incompatible Java 25 runtime
+from being selected. Fabric Loader 0.17.0 and later retains the existing open
+upper bound so compatible newer loaders may use Java 25.
+
+The boundary is based on the loader-published dependencies: Fabric Loader
+0.16.12 uses ASM 9.7.1, while 0.17.0 upgrades to ASM 9.8 with Java 25
+class-file support.
+
+The constraint is applied only in automatic selection. Explicit Java choices
+and user-defined Java ranges remain authoritative, preserving the existing
+escape hatches for advanced users.
+
+## Verification
+
+- Confirm the 1.21.4/Fabric 0.16.12 requirement becomes `[21.0, 25.0)`.
+- Confirm Java 25 is skipped and Java 21 is selected when both are configured.
+- Confirm a Fabric Loader version at or above 0.17.0 retains its existing
+  Java-version range.
+- Build the solution and review the final diff for scope and regressions.