fix(win): export embedded LibMTRSim symbols from the plugin DLL on MSVC

The plugin-build shim hard-coded LIBMTRSIM_EXPORT to empty. On Unix that is
fine (default visibility exports the symbols), but on MSVC nothing is exported
from the plugin DLL, so external consumers (MTRSimUnitTest, the Python
bindings) fail to link mtrsim:: free functions (readODFComponents,
readODFMetadata, tryReadFixtureVersion) with LNK2019/LNK1120. Key the macro on
MTRSim_EXPORTS (dllexport when building the plugin, dllimport when consuming),
matching the plugin's own export macro. macOS build + 42/42 tests unaffected.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: imikejackson

src/MTRSim/libmtrsim_export.h

@@ -1,7 +1,25 @@
 /**
-This file is here because we are including all the LibMTRSim sources into the
-simplnx plugin. In order to do that, this file with this define needs to be
-here.
-*/
+ * The LibMTRSim sources are compiled directly into the SIMPLNX MTRSim plugin
+ * shared library (rather than a standalone libMTRSim). This shim defines the
+ * LibMTRSim export macro so the LibMTRSim symbols are exported from / imported
+ * into the plugin DLL exactly like the plugin's own symbols. Without this, on
+ * MSVC the LibMTRSim symbols (e.g. mtrsim::readODFComponents) are not exported
+ * from the plugin DLL and external consumers -- the unit-test executable and
+ * the Python bindings -- fail to link with LNK2019 / LNK1120. On non-Windows
+ * the symbols use default visibility (the prior behavior).
+ *
+ * CMake defines `MTRSim_EXPORTS` while building the plugin target.
+ */
+#pragma once
 
-#define LIBMTRSIM_EXPORT
+#ifndef LIBMTRSIM_EXPORT
+#ifdef _WIN32
+#ifdef MTRSim_EXPORTS
+#define LIBMTRSIM_EXPORT __declspec(dllexport)
+#else
+#define LIBMTRSIM_EXPORT __declspec(dllimport)
+#endif
+#else
+#define LIBMTRSIM_EXPORT __attribute__((visibility("default")))
+#endif
+#endif