Version 2.1 release prep

[LinuxJedi]

• Bump to 2.1
• Fix paths so distcheck works
• Fix issues with C++ building
• Add CI for C++ building

[Copilot]

Pull request overview

Prepares the wolfPKCS11 codebase for the 2.1 release by bumping version metadata, improving distcheck/VPATH behavior, and tightening C++ compatibility via code changes and CI.

Changes:

• Bump project version references to 2.1.0 across headers/build/doc tooling.
• Adjust tests/examples/internal code to compile cleanly as C++ (casts, avoiding C++ keywords, avoiding compound literals).
• Improve test execution under make distcheck (create build-tree store/, export srcdir, clean test artifacts) and add a CI job for C++ builds.

Reviewed changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
wolfpkcs11/version.h	Version string/hex bump for library version reporting.
tests/pkcs11test.c	C++-compat fixes (casts, avoid compound literal, CK_ULONG casts, rename template).
tests/pkcs11str.c	C++-compat fixes for attribute length initializers.
tests/pkcs11mtt.c	C++-compat fixes (CK_ATTRIBUTE_TYPE cast, CK_ULONG casts).
tests/object_id_uniqueness_test.c	Rename template to tmpl for C++ keyword compatibility.
src/internal.c	C++-compat fixes (void* casts for XMALLOC, enum-from-int casts for hash type).
README.md	Add 2.1 release notes section.
Makefile.am	Ensure store/ exists for tests; export srcdir; clean up test artifacts for distcheck.
examples/slot_info.c	Add C++-required cast for malloc.
examples/obj_list.c	Refactor print helpers to take CK_VOID_PTR and cast internally for C++ compatibility.
examples/mech_info.c	Add C++-required cast for malloc.
examples/examples.test	Use $srcdir for distributed RSA DER file in VPATH builds.
doc/formats/pdf/Doxyfile	Bump Doxygen project version to 2.1.0.
doc/formats/html/Doxyfile	Bump Doxygen project version to 2.1.0.
configure.ac	Bump autotools project version to 2.1.0.
CMakeLists.txt	Bump CMake project version to 2.1.0.
.github/workflows/unit-test.yml	Add CI job that builds/tests with g++ and -Werror=narrowing.

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