Fix pack '.' format to handle negative positions

The '.' format in pack() was throwing an error for negative positions,
blocking 51 tests in pack.t. Perl treats negative positions as 0
(truncate to beginning of string).

Changes:
- Modified ControlPackHandler.java to treat negative positions as 0
- Removed explicit error throwing for negative positions

Test Results:
- pack.t: 14673 → 14724 tests running (all tests now run!)
- No longer blocks at line 1949 with negative position error
- 13811/14724 tests passing (93.8% pass rate)

Example that now works:
pack('(W)5 (. @2 a)', 0x301..0x305, -3, 'a')

Impact: +51 tests unblocked in pack.t

Author: fglock

src/main/java/org/perlonjava/operators/pack/ControlPackHandler.java

@@ -50,10 +50,9 @@ public int pack(List<RuntimeScalar> values, int valueIndex, int count, boolean h
                 valueIndex++;
                 int targetPos = (int) posValue.getDouble();
 
-                // DEBUG: '.' format - current size: " + output.size() + ", target position: " + targetPos
-
+                // Handle negative positions: treat as 0 (truncate to beginning)
                 if (targetPos < 0) {
-                    throw new PerlCompilerException("pack: negative position for '.'");
+                    targetPos = 0;
                 }
 
                 int currentSize = output.size();