Use fetched Catch2 and simplify vcpkg dependencies

[huangminghuang]

Summary

This removes Catch2 and LLVM from the vcpkg manifest/custom registry path. softfloat is now the only vcpkg dependency for eos-vm.

Catch2 is only used when ENABLE_TESTS=ON. The wire-sys-vm registry port explicitly configures eos-vm with tests disabled, so depending on Catch2 during vcpkg packaging was unnecessary in the first place. On top of that, eos-vm intentionally uses an old Catch2 revision, matching the original eos-vm external/Catch2 submodule commit. That old Catch2 version is not shared by the other repos in the Wire system. Maintaining an independent vcpkg Catch2 port, with a no-posix-signals configuration used only by this repo's tests, adds registry maintenance without providing reuse value.

Instead, eos-vm tests now use CMake FetchContent pinned to the historical Catch2 commit:

• 9c741fe96073ed620ffc032afbed1f3c789d2b68

This keeps the Catch2 choice local to eos-vm tests and lets the shared Wire vcpkg registry stop carrying repo-specific Catch2 behavior.

LLVM is removed for the same reason: it is not used by the sys-vm library or packaging path. The VM JIT is hand-written machine-code emission, not LLVM. The only local LLVM-related use was the fuzz-test setup, so the fuzz CMake now checks whether the active compiler supports -fsanitize=fuzzer,address directly instead of forcing every vcpkg consumer to install a large LLVM package.

Changes

• Remove catch2 and llvm from vcpkg.json; softfloat is now the only vcpkg dependency.
• Trim vcpkg-configuration.json so the Wire custom registry only owns softfloat for this repo.
• Remove global find_package(Catch2) and find_package(LLVM) from dependency setup.
• Use FetchContent for the pinned Catch2 test dependency.
• Keep CATCH_CONFIG_NO_POSIX_SIGNALS on test targets.
• Update direct signal-handler tests to pass explicit allocator arguments and document the empty-memory-range behavior.
• Update fuzz test wiring to check compiler support for -fsanitize=fuzzer,address instead of depending on a vcpkg LLVM package.
• Fix the fuzz harness for the current sysio::vm API and add a VM watchdog so long-running wasm corpus inputs are rejected cleanly.

Validation

• Fresh configure with ENABLE_TESTS=OFF passes; vcpkg resolves only softfloat plus helper ports.
• Fresh configure/build with tests enabled passed for the main test targets during local validation.
• Signal/trap smoke tests passed with the fetched Catch2 revision.
• Fuzz support was tested with clang-18:
  • compiler probe for -fsanitize=fuzzer,address passes
  • LLVMFuzzer-wasm_fuzz_test builds
  • ctest -R wasm_libfuzzer passes with the full current corpus

Notes

No changes are required in wire-vcpkg-registry for this PR, but after this lands the custom Catch2 port there should be removable if no other repo depends on it.