Debugger: Remove opcode-fusion display and fix cpu step size units

DisassemblyManager used to fuse lui+addiu/load/store into single pseudo-
instructions ("li", fused loads/stores) for display. This only applied to a
handful of opcodes, complicated DisassemblyManager, and was the root cause of
a stepping bug: Core_PerformCPUStep's Into/Over cases treated stepSize as a
byte count, while the WebSocket cpu.stepInto handler computed it as an
instruction count (needed to step over a whole fused macro in one go) - so a
plain, non-fused stepInto silently executed zero instructions.

Removed the fusion logic entirely (DisassemblyMacro, DISTYPE_MACRO) - every
disassembly line is now exactly one 4-byte instruction. With that,
"how many instructions does this line span" is always 1, so the
getInstructionSizeAt() byte-size queries in the legacy Windows and ImGui
debuggers are gone too; step requests just pass 1. Core_RequestCPUStep's
stepSize is now consistently in instructions everywhere.

Also fixes the PPSSPPHeadless build, broken since 0ed1f3e added
OpenWebDebugger() (which calls System_LaunchUrl) without a headless stub.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Hqm11k99viLfbJm2MkH4BH

Windows/Debugger/Debugger_Disasm.cpp

@@ -206,8 +206,7 @@ void CDisasm::step(CPUStepType stepType) {
 	ptr->setDontRedraw(true);
 	lastTicks_ = CoreTiming::GetTicks();
 
-	u32 stepSize = ptr->getInstructionSizeAt(cpu->GetPC());
-	Core_RequestCPUStep(stepType, stepSize);
+	Core_RequestCPUStep(stepType, 1);
 }
 
 void CDisasm::runToLine() {