Add TRACY_PLATFORM_HEADER hook for unsupported platforms.

Extension point so private/unsupported platforms can plug in their own implementations of the kernel/libc primitives Tracy depends on, without patching the `#if`/`#elif` chains.

Projects supply a platform header via `-DTRACY_PLATFORM_HEADER="\"my_platform.h\""` at build time. Tracy includes it in any TU that needs the hooks. The header toggles per-category `TRACY_HAS_CUSTOM_*` macros and declares matching `tracy::Platform*` functions.

Available hooks:

- `TRACY_HAS_CUSTOM_THREAD_ID` → `PlatformGetThreadId`
- `TRACY_HAS_CUSTOM_USER_INFO` → `PlatformGetHostname`, `PlatformGetUserLogin`, `PlatformGetUserFullName`
- `TRACY_HAS_CUSTOM_SAFE_COPY` → `PlatformSafeMemcpy`
- `TRACY_HAS_CUSTOM_ALLOCATOR` → `PlatformMalloc`, `PlatformFree`, `PlatformRealloc`, `PlatformAllocatorInit`, `PlatformAllocatorThreadInit`, `PlatformAllocatorFinalize`, `PlatformAllocatorThreadFinalize`

Each hook is wired as the first arm of its respective `#if`/`#elif` chain, so existing supported platforms are unaffected.

Template files in `examples/CustomPlatform/` and a new subsection in `manual/tracy.tex` document the mechanism.

public/common/TracySystem.cpp

@@ -51,6 +51,10 @@
 
 #include "TracySystem.hpp"
 
+#ifdef TRACY_PLATFORM_HEADER
+#  include TRACY_PLATFORM_HEADER
+#endif
+
 #if defined _WIN32
 extern "C" typedef HRESULT (WINAPI *t_SetThreadDescription)( HANDLE, PCWSTR );
 extern "C" typedef HRESULT (WINAPI *t_GetThreadDescription)( HANDLE, PWSTR* );
@@ -69,7 +73,9 @@ namespace detail
 
 TRACY_API uint32_t GetThreadHandleImpl()
 {
-#if defined _WIN32
+#if defined TRACY_HAS_CUSTOM_THREAD_ID
+    return PlatformGetThreadId();
+#elif defined _WIN32
     static_assert( sizeof( decltype( GetCurrentThreadId() ) ) <= sizeof( uint32_t ), "Thread handle too big to fit in protocol" );
     return uint32_t( GetCurrentThreadId() );
 #elif defined __APPLE__
@@ -341,7 +347,9 @@ TRACY_API const char* GetEnvVar( const char* name )
 
 TRACY_API const char* GetUserLogin()
 {
-#if defined _WIN32
+#if defined TRACY_HAS_CUSTOM_USER_INFO
+    return PlatformGetUserLogin();
+#elif defined _WIN32
 #  if defined TRACY_WIN32_NO_DESKTOP
     return "(?)";
 #  else
@@ -363,7 +371,9 @@ TRACY_API const char* GetUserLogin()
 
 TRACY_API const char* GetUserFullName()
 {
-#if defined _WIN32
+#if defined TRACY_HAS_CUSTOM_USER_INFO
+    return PlatformGetUserFullName();
+#elif defined _WIN32
     static char buf[1024];
     ULONG size = sizeof( buf );
     if( GetUserNameExA( NameDisplay, buf, &size ) ) return buf;