Nixpkgs builds emscripten with the generic llvmPackages release LLVM,
not the llvm-project main-branch commit that emsdk pins. Release LLVM
(<= 22.x) generates calls to _Unwind_CallPersonality in catch handlers.
Emscripten's bundled libunwind no longer defines _Unwind_CallPersonality:
llvm-project PR #209282 moved the helper into libc++abi under the name
__gxx_wasm_personality_v0. Restore the old entry point as a forwarder to
__gxx_wasm_personality_v0, so that -fwasm-exceptions links.
Assisted-by: Claude Code (Claude Fable 5)