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 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Hqm11k99viLfbJm2MkH4BH

Core/Debugger/WebSocket/DisasmSubscriber.cpp

@@ -78,8 +78,6 @@ void WebSocketDisasmState::WriteDisasmLine(JsonWriter &json, const DisassemblyLi
 	json.pushDict();
 	if (l.type == DISTYPE_OPCODE)
 		json.writeString("type", "opcode");
-	else if (l.type == DISTYPE_MACRO)
-		json.writeString("type", "macro");
 	else if (l.type == DISTYPE_DATA)
 		json.writeString("type", "data");
 	else if (l.type == DISTYPE_OTHER)