emscripten_dlopen deadlock when re-opening already-loaded library

## Version of emscripten/emsdk

```
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 5.0.0 (a7c5deabd7c88ba1c38ebe988112256775f944c6)
clang version 23.0.0git (https:/github.com/llvm/llvm-project 358db292cc6a9a8a5448a296f643312289f328d7)
Target: wasm32-unknown-emscripten
Thread model: posix
InstalledDir: ~/code/emsdk/upstream/bin
```

## Bug description

`emscripten_dlopen()` deadlocks when opening a library that is already loaded. The `find_existing()` early-return path at [`system/lib/libc/dynlink.c:580-586`](https://github.com/emscripten-core/emscripten/blob/d4c1c6e1d37bf38e84cca063753a462143b7af9a/system/lib/libc/dynlink.c#L580-L586) acquires `do_write_lock()` but never calls `do_write_unlock()`:

```c
do_write_lock(); // L580: lock acquired
char buf[2*NAME_MAX+2];
filename = find_dylib(buf, filename, sizeof buf);
struct dso* p = find_existing(filename); // L583
if (p) {
onsuccess(user_data, p); // L585: callback called
return; // L586: MISSING do_write_unlock()
}
```

Every other path in `emscripten_dlopen` properly releases the lock:
- L590: `do_write_unlock()` before `onerror()` callback
- L441: `do_write_unlock()` in `dlopen_onsuccess()` callback
- L449: `do_write_unlock()` in `dlopen_onerror()` callback

The leaked lock causes the next `dlopen` call to deadlock. Only affects `-pthread` builds where `do_write_lock()` is a real mutex (lines 97-99 define them as no-ops in non-`_REENTRANT` builds).

## Reproducing

Save `side.c`:

```c
int foo = 42;
```

Save `test.c`:

```c
#include
#include
#include
#include
#include

static int callback_count = 0;

void onsuccess_third(void *user_data, void *handle) {
callback_count++;
printf("Third dlopen succeeded (callback_count=%d)\n", callback_count);
printf("PASS: All three dlopen calls succeeded without deadlock\n");
exit(0);
}

void onerror_third(void *user_data) {
printf("Third dlopen failed: %s\n", dlerror());
exit(1);
}

void onsuccess_second(void *user_data, void *handle) {
callback_count++;
printf("Second dlopen succeeded (callback_count=%d)\n", callback_count);
printf("Attempting third dlopen of same library...\n");
emscripten_dlopen("libside.so", RTLD_NOW, NULL, onsuccess_third, onerror_third);
}

void onerror_second(void *user_data) {
printf("Second dlopen failed: %s\n", dlerror());
exit(1);
}

void onsuccess_first(void *user_data, void *handle) {
callback_count++;
printf("First dlopen succeeded (callback_count=%d)\n", callback_count);
printf("Attempting second dlopen of same library...\n");
emscripten_dlopen("libside.so", RTLD_NOW, NULL, onsuccess_second, onerror_second);
}

void onerror_first(void *user_data) {
printf("First dlopen failed: %s\n", dlerror());
exit(1);
}

int main() {
printf("Test: dlopen same library three times (deadlock bug)\n");
emscripten_dlopen("libside.so", RTLD_NOW, NULL, onsuccess_first, onerror_first);
printf("Returning from main, waiting for callbacks...\n");
return 99;
}
```

Three `emscripten_dlopen` calls are needed to trigger this:
1. **1st call** loads the library fresh (async path, lock properly released in `dlopen_onsuccess`)
2. **2nd call** finds the library via `find_existing()`, leaks the lock (bug)
3. **3rd call** tries to acquire the leaked lock and deadlocks

## Failing command line in full

```
emcc side.c -o libside.so -sSIDE_MODULE -pthread
emcc test.c libside.so -sMAIN_MODULE=2 -pthread -sPROXY_TO_PTHREAD -sEXIT_RUNTIME -o test.js
node test.js
```

## Runtime output

```
Test: dlopen same library three times (deadlock bug)
Returning from main, waiting for callbacks...
First dlopen succeeded (callback_count=1)
Attempting second dlopen of same library...
Second dlopen succeeded (callback_count=2)
Attempting third dlopen of same library...
Aborted(Assertion failed: at || own != __pthread_self()->tid && "pthread mutex deadlock detected", at: /emsdk/emscripten/system/lib/libc/musl/src/thread/pthread_mutex_timedlock.c,95,__pthread_mutex_timedlock)
```

## Full link command and output with `-v` appended

Side module compile (`emcc side.c -o libside.so -sSIDE_MODULE -pthread -v`):

```
~code/emsdk/upstream/bin/clang -target wasm32-unknown-emscripten -fignore-exceptions -fPIC -fvisibility=default -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --sysroot=~code/emsdk/upstream/emscripten/cache/sysroot -D__EMSCRIPTEN_SHARED_MEMORY__=1 -DEMSCRIPTEN -Xclang -iwithsysroot/include/fakesdl -Xclang -iwithsysroot/include/compat -pthread -v -c side.c -o /var/folders/wk/3v2syqm519z2kg7nqf4xtgy80000gn/T/emscripten_temp_vntpzynh/side.o
emcc: warning: dynamic linking + pthreads is experimental [-Wexperimental]
~code/emsdk/upstream/bin/wasm-ld -o libside.so --import-memory --shared-memory --strip-debug --export-dynamic --export=__wasm_call_ctors --export=_emscripten_tls_init --export-if-defined=__start_em_asm --export-if-defined=__stop_em_asm --export-if-defined=__start_em_lib_deps --export-if-defined=__stop_em_lib_deps --export-if-defined=__start_em_js --export-if-defined=__stop_em_js --export-if-defined=main --export-if-defined=__main_argc_argv --export-if-defined=fflush --experimental-pic --unresolved-symbols=import-dynamic -shared --stack-first --whole-archive /var/folders/wk/3v2syqm519z2kg7nqf4xtgy80000gn/T/emscripten_temp_vntpzynh/side.o -L~code/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/pic -L~code/emsdk/upstream/emscripten/src/lib ~code/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/pic/crtbegin.o --no-whole-archive -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr
```

Main module link (`emcc test.c libside.so -sMAIN_MODULE=2 -pthread -sPROXY_TO_PTHREAD -sEXIT_RUNTIME -o test.js -v`):

```
~code/emsdk/upstream/bin/clang -target wasm32-unknown-emscripten -fignore-exceptions -fPIC -fvisibility=default -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr --sysroot=~code/emsdk/upstream/emscripten/cache/sysroot -D__EMSCRIPTEN_SHARED_MEMORY__=1 -DEMSCRIPTEN -Xclang -iwithsysroot/include/fakesdl -Xclang -iwithsysroot/include/compat -pthread -v -c test.c -o /var/folders/wk/3v2syqm519z2kg7nqf4xtgy80000gn/T/emscripten_temp_edrh4eqr/test_dynlink_unlock.o
emcc: warning: dynamic linking + pthreads is experimental [-Wexperimental]
~code/emsdk/upstream/bin/wasm-ld -o test.wasm /var/folders/wk/3v2syqm519z2kg7nqf4xtgy80000gn/T/tmpiiro9j26libemscripten_js_symbols.so --import-memory --shared-memory --strip-debug -Bdynamic --export=__stack_pointer --export=emscripten_stack_get_end --export=emscripten_stack_get_free --export=emscripten_stack_get_base --export=emscripten_stack_get_current --export=emscripten_stack_init --export=_emscripten_stack_alloc --export=_emscripten_thread_free_data --export=_emscripten_thread_crashed --export=_emscripten_dlsync_self --export=_emscripten_dlsync_self_async --export=_emscripten_proxy_dlsync --export=_emscripten_proxy_dlsync_async --export=__dl_seterr --export=__funcs_on_exit --export=__wasm_call_ctors --export=_emscripten_tls_init --export=setThrew --export=_emscripten_stack_restore --export=_emscripten_find_dylib --export=strerror --export=emscripten_get_sbrk_ptr --export=__heap_base --export=calloc --export=_emscripten_thread_init --export=emscripten_stack_set_limits --export=_emscripten_thread_exit --export-if-defined=__start_em_asm --export-if-defined=__stop_em_asm --export-if-defined=__start_em_lib_deps --export-if-defined=__stop_em_lib_deps --export-if-defined=__start_em_js --export-if-defined=__stop_em_js --export-if-defined=main --export-if-defined=__main_argc_argv --export-if-defined=fflush --export-if-defined=__memory_base --export-if-defined=__table_base --export-if-defined=emscripten_builtin_memalign --export-if-defined=pthread_self --experimental-pic --unresolved-symbols=import-dynamic --export-table --growable-table -z stack-size=65536 --no-growable-memory --initial-memory=16777216 --entry=_emscripten_proxy_main --stack-first --table-base=1 /var/folders/wk/3v2syqm519z2kg7nqf4xtgy80000gn/T/emscripten_temp_edrh4eqr/test_dynlink_unlock.o libside.so -L~code/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/pic -L~code/emsdk/upstream/emscripten/src/lib ~code/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/pic/crtbegin.o ~code/emsdk/upstream/emscripten/cache/sysroot/lib/wasm32-emscripten/pic/crt1_proxy_main.o -lGL-mt-getprocaddr -lal -lhtml5 -lstubs-debug -lc-mt-debug -ldlmalloc-mt-debug -lcompiler_rt-mt -lc++-debug-mt-noexcept -lc++abi-debug-mt-noexcept -lsockets-mt -mllvm -combiner-global-alias-analysis=false -mllvm -enable-emscripten-sjlj -mllvm -disable-lsr
~code/emsdk/node/22.16.0_64bit/bin/node ~code/emsdk/upstream/emscripten/tools/compiler.mjs -
```

## Suggested fix

```diff
if (p) {
+ do_write_unlock();
onsuccess(user_data, p);
return;
}
```