fix: gate -lopfs.js behind pthreads to fix OPFS crash

[kbatchu]

Summary

Fixes the OPFS UNREACHABLE crash at opfs_backend.cpp:292 by ensuring -lopfs.js is only linked when pthreads are enabled.

• Move -lopfs.js and OPFS exported functions (_wasmfs_create_opfs_backend, _wasmfs_create_directory) inside the NOT __SINGLE_THREADED__ guard
• Single-threaded browser builds still get WasmFS (in-memory) but no longer link the broken OPFS backend
• Multi-threaded browser builds get OPFS with correct pthread pairing

Root cause

-lopfs.js is linked unconditionally in the browser WasmFS build (the else() branch), but -pthread is only added when NOT __SINGLE_THREADED__. The Emscripten OPFS JS backend internally calls _emscripten_proxy_finish, which is only defined with pthreads. Without it, wasmfsOPFSProxyFinish compiles as a no-op, so at runtime the OPFS backend reads childType=0 and hits:

Unexpected child type
UNREACHABLE executed at opfs_backend.cpp:292

What changes

CMakeLists.txt — In the BUILD_WASM / browser-WasmFS branch:

Before	After
-lopfs.js linked unconditionally	-lopfs.js only linked when NOT __SINGLE_THREADED__
OPFS exports always present	OPFS exports only in multi-threaded build
allocateUTF8 always exported	allocateUTF8 only exported when OPFS is available

The companion JS-side fixes (WasmFS C API usage, runtime detection, OPFS tests) are already merged in ladybug-wasm — see commits 6305367 and c495097.

Test plan

• Multi-threaded browser build (SINGLE_THREADED=false): verify OPFS mounts and persists data
• Single-threaded browser build (SINGLE_THREADED=true): verify build succeeds, mountOpfs() returns a clear error instead of crashing
• Node.js build (WASM_NODEFS=true): verify no regression (unaffected code path)
• WASM test build (BUILD_TESTS=true): verify no regression (NODERAWFS path)

Ref: #305