fix: guard against uint32 underflow in memset/memcpy and strings unit

When size=0 was passed to memset() or memcpy(), the loop bound
`0 to size-1` underflowed to $FFFFFFFF, iterating ~4 billion times
and causing an immediate page fault.

Added `if size = 0 then exit` guards to both memset() and memcpy()
in util.pas.

Also guarded all `for i:=0 to X-1` loops in strings.pas where X
could be 0 on empty string inputs:
- stringToUpper: wrap loop in `if stringSize > 0`
- stringToLower: wrap loop in `if stringSize > 0`
- stringEquals: early exit when both sizes are 0
- stringToInt: early exit when string is empty

Additionally fixed all previously identified bugs in strings.pas:
- hexStringToInt: guard against empty string underflow
- stringConcat: cache stringSize calls to avoid redundant O(n) scans
- stringTrim: clamp length to source string size
- stringSub: bounds check start/size against source length
- stringReplace: complete rewrite - was using stringEquals() for
  substring matching (only matched at string end), dropped prefix
  chars, had dangling assignment overwriting valid result with
  uninitialized pointer, and had unused variable
- stringIndexOf: rewritten to use new stringMatchAt() helper for
  correct substring matching
- stringContains: simplified to delegate to fixed stringIndexOf(),
  also fixing uint32 underflow when sub was empty
- stringToInt: removed always-true `v >= 0` check on uint32
- intToString: implemented (was a stub returning ' ')
- Added stringMatchAt() helper for prefix comparison
- Added comprehensive UnitTest procedure with 77 assertions
  including adversarial empty/nil/boundary inputs

src/include/util.pas

@@ -414,6 +414,7 @@ var
 
 begin
     //push_trace('util.memset');
+    if size = 0 then exit;
     for i:=0 to size-1 do begin
         loc:= puint8(location + i);
         loc^:= value;
@@ -428,6 +429,7 @@ var
 
 begin
     //push_trace('util.memcpy');
+    if size = 0 then exit;
     for i:=0 to size-1 do begin
         src:= puint8(source + i);
         dst:= puint8(dest + i);

src/kernel.pas

@@ -251,6 +251,9 @@ begin
      { Initialize visual terminal (registers with desktop search) }
      vterminal.init;
 
+     { Run unit tests }
+     strings.UnitTest;
+
      { Main render loop }
      syslog.logln('KERNEL', 'Entering main render loop.');
      tracer.push_trace('kmain.MAINLOOP');