feat(core): add Arena overload for lookupFunction and clean CHANGELOG

Author: andrestubbe

docs/CHANGELOG.md

@@ -18,8 +18,6 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/).
 - **Hero Demo & Automation**:
   - Standalone runner `run-demo.bat` for quick verification.
   - Enhanced `Demo.java` testing both traditional JNI and Java 21+ FFM Linkers.
-- **Architectural Roadmap**:
-  - Added hidden `.TODO.md` outlining the FastJava FFM migration path.
 
 ### Changed
 - Upgraded Java language baseline from 17 to **21 LTS** (`--enable-preview`).

src/main/java/fastcore/FastCore.java

@@ -229,6 +229,21 @@ public static MethodHandle lookupFunction(String libraryName, String functionNam
         return lookupFunction(libraryName, functionName, descriptor, null, Arena.global());
     }
 
+    /**
+     * Looks up and links an exported native C function using a custom arena and default classloader.
+     *
+     * @param libraryName  Logical library name
+     * @param functionName Exported C function symbol
+     * @param descriptor   Function signature descriptor
+     * @param arena        Custom arena scoping the symbol lookup
+     * @return An invokable {@link MethodHandle}
+     * @throws Exception If the library or symbol cannot be resolved
+     */
+    public static MethodHandle lookupFunction(String libraryName, String functionName,
+                                              FunctionDescriptor descriptor, Arena arena) throws Exception {
+        return lookupFunction(libraryName, functionName, descriptor, null, arena);
+    }
+
     /**
      * Zero-overhead conversion of a 64-bit native memory address (e.g. from {@code FastPointer})
      * into an unbounded {@link MemorySegment}.