libghostty: add cpp-vt-stream example and fix C++ header compatibility

Add a cpp-vt-stream example that verifies libghostty headers compile
cleanly in C++ mode. The example is a simplified C++ port of
c-vt-stream.

The headers used the C idiom `typedef struct Foo* Foo` for opaque
handles, which is invalid in C++ because struct tags and typedefs
share the same namespace. Fix all 12 opaque handle typedefs across the
headers to use a distinct struct tag with an Impl suffix, e.g.
`typedef struct GhosttyTerminalImpl* GhosttyTerminal`. This is a
source-compatible change for existing C consumers since the struct
tags were never referenced directly.

include/ghostty/vt/formatter.h

@@ -111,7 +111,7 @@ typedef struct {
  *
  * @ingroup formatter
  */
-typedef struct GhosttyFormatter* GhosttyFormatter;
+typedef struct GhosttyFormatterImpl* GhosttyFormatter;
 
 /**
  * Options for creating a terminal formatter.

include/ghostty/vt/key/encoder.h

@@ -22,7 +22,7 @@
  *
  * @ingroup key
  */
-typedef struct GhosttyKeyEncoder *GhosttyKeyEncoder;
+typedef struct GhosttyKeyEncoderImpl *GhosttyKeyEncoder;
 
 /**
  * Kitty keyboard protocol flags.

include/ghostty/vt/key/event.h

@@ -21,7 +21,7 @@
  *
  * @ingroup key
  */
-typedef struct GhosttyKeyEvent *GhosttyKeyEvent;
+typedef struct GhosttyKeyEventImpl *GhosttyKeyEvent;
 
 /**
  * Keyboard input event types.

include/ghostty/vt/mouse/encoder.h

@@ -23,7 +23,7 @@
  *
  * @ingroup mouse
  */
-typedef struct GhosttyMouseEncoder *GhosttyMouseEncoder;
+typedef struct GhosttyMouseEncoderImpl *GhosttyMouseEncoder;
 
 /**
  * Mouse tracking mode.

include/ghostty/vt/mouse/event.h

@@ -20,7 +20,7 @@
  *
  * @ingroup mouse
  */
-typedef struct GhosttyMouseEvent *GhosttyMouseEvent;
+typedef struct GhosttyMouseEventImpl *GhosttyMouseEvent;
 
 /**
  * Mouse event action type.

include/ghostty/vt/osc.h

@@ -21,7 +21,7 @@
  *
  * @ingroup osc
  */
-typedef struct GhosttyOscParser *GhosttyOscParser;
+typedef struct GhosttyOscParserImpl *GhosttyOscParser;
 
 /**
  * Opaque handle to a single OSC command.
@@ -31,7 +31,7 @@ typedef struct GhosttyOscParser *GhosttyOscParser;
  *
  * @ingroup osc
  */
-typedef struct GhosttyOscCommand *GhosttyOscCommand;
+typedef struct GhosttyOscCommandImpl *GhosttyOscCommand;
 
 /** @defgroup osc OSC Parser
  *

include/ghostty/vt/render.h

@@ -86,21 +86,21 @@ extern "C" {
  *
  * @ingroup render
  */
-typedef struct GhosttyRenderState* GhosttyRenderState;
+typedef struct GhosttyRenderStateImpl* GhosttyRenderState;
 
 /**
  * Opaque handle to a render-state row iterator.
  *
  * @ingroup render
  */
-typedef struct GhosttyRenderStateRowIterator* GhosttyRenderStateRowIterator;
+typedef struct GhosttyRenderStateRowIteratorImpl* GhosttyRenderStateRowIterator;
 
 /**
  * Opaque handle to render-state row cells.
  *
  * @ingroup render
  */
-typedef struct GhosttyRenderStateRowCells* GhosttyRenderStateRowCells;
+typedef struct GhosttyRenderStateRowCellsImpl* GhosttyRenderStateRowCells;
 
 /**
  * Dirty state of a render state after update.

include/ghostty/vt/sgr.h

@@ -55,7 +55,7 @@ extern "C" {
  *
  * @ingroup sgr
  */
-typedef struct GhosttySgrParser* GhosttySgrParser;
+typedef struct GhosttySgrParserImpl* GhosttySgrParser;
 
 /**
  * SGR attribute tags.

include/ghostty/vt/terminal.h

@@ -159,7 +159,7 @@ extern "C" {
  *
  * @ingroup terminal
  */
-typedef struct GhosttyTerminal* GhosttyTerminal;
+typedef struct GhosttyTerminalImpl* GhosttyTerminal;
 
 /**
  * Terminal initialization options.