fix(native): vtable dispatch for virtual method calls in expression context

EmitMethodCallExpr was always emitting a direct `callq` for method calls,
ignoring VTableSlot.  Virtual calls like B.GetVal() (where B's declared
type is an abstract base) produced a direct call to the base's method
symbol, which is never emitted for abstract methods — causing link errors.

Add vtable dispatch (load vtable ptr from Self, index by VTableSlot,
indirect call) in both the <=6 and >6 arg paths of EmitMethodCallExpr,
matching what EmitMethodCallStmt already does.

This resolves the last undefined references (Streams_TInputStream_Read,
Streams_TOutputStream_Write) when native-compiling the compiler itself.
The native-compiled compiler now links and runs.

compiler/src/main/pascal/blaise.codegen.native.x86_64.pas

@@ -5282,7 +5282,14 @@ begin
     for I := UserSlots - 1 downto 0 do
       Self.Emit(#9'popq ' + SysVArgRegs64[I + 1]);
     Self.Emit(#9'movq %r10, %rdi');
-    Self.Emit(#9'callq ' + Sym);
+    if MD.VTableSlot >= 0 then
+    begin
+      Self.Emit(#9'movq (%rdi), %rax');
+      Self.Emit(Format(#9'movq %d(%%rax), %%rax', [(MD.VTableSlot + 1) * 8]));
+      Self.Emit(#9'callq *%rax');
+    end
+    else
+      Self.Emit(#9'callq ' + Sym);
   end
   else
   begin
@@ -5343,7 +5350,14 @@ begin
       Self.Emit(Format(#9'movq %d(%%rsp), %s', [I * 8, SysVArgRegs64[I]]));
 
     Self.Emit(Format(#9'addq $%d, %%rsp', [6 * 8]));
-    Self.Emit(#9'callq ' + Sym);
+    if MD.VTableSlot >= 0 then
+    begin
+      Self.Emit(#9'movq (%rdi), %rax');
+      Self.Emit(Format(#9'movq %d(%%rax), %%rax', [(MD.VTableSlot + 1) * 8]));
+      Self.Emit(#9'callq *%rax');
+    end
+    else
+      Self.Emit(#9'callq ' + Sym);
     Self.Emit(Format(#9'addq $%d, %%rsp', [CleanUp]));
   end;
 end;

compiler/src/test/pascal/cp.test.e2e.native.pas

@@ -287,6 +287,9 @@ type
 
     { Const array data emission + ConstArraySymbol reference }
     procedure TestRun_Native_ConstArray_StringElements;
+
+    { Virtual method dispatch in expression context }
+    procedure TestRun_Native_VirtualDispatch_Expr;
   end;
 
 implementation
@@ -4330,6 +4333,28 @@ begin
     'ax' + LE + 'bx' + LE + 'cx' + LE, 0);
 end;
 
+procedure TE2ENativeTests.TestRun_Native_VirtualDispatch_Expr;
+begin
+  AssertRunsOnBoth(
+    'program T;'
+    + 'type '
+    + '  TBase = class '
+    + '    function GetVal(): Integer; virtual; abstract; '
+    + '  end; '
+    + '  TChild = class(TBase) '
+    + '    function GetVal(): Integer; override; '
+    + '  end; '
+    + 'function TChild.GetVal(): Integer; '
+    + 'begin Result := 42 end; '
+    + 'var B: TBase; '
+    + 'begin '
+    + '  B := TChild.Create; '
+    + '  WriteLn(B.GetVal()); '
+    + '  B.Free '
+    + 'end.',
+    '42' + LE, 0);
+end;
+
 initialization
   RegisterTest(TE2ENativeTests);