libSceAmpr: _Cnd_signal and _Cnd_init_with_name are stubs after #3079; fix branch ready #3342

_Cnd_signal and _Cnd_init_with_name (libSceAmpr, Export.cpp) are NotImplemented_nid_no_patch stubs now that #3079 gave libc the Dinkumware _Cnd_* family. Deathloop (PPSA01670), Minecraft Legends and GTA III Definitive Edition import them.

Fix branch ready: screamm:feat/ampr-cnd-signal (e21484de, one commit on main a4b0b16b). I cannot open it as a PR because of the intake check; anyone eligible is welcome to carry it, keeping the commit as is.

• _Cnd_signal(pthread_cond**) uses the pointer-to-slot ABI the feat(libc): implement Dinkumware mutex and condition exports #3079 review established and a new libc helper SignalCondition_nid_no_patch (in Threads.cpp) that wakes one waiter of the condition objects libc's _Cnd_init creates and returns _Thrd_success, like the merged _Cnd_broadcast; bad handles throw the same invalid_argument as libc. _Cnd_init_with_name(pthread_cond**, const char*) calls libc's _Cnd_init and ignores the name, following fpPS4's reverse-engineered PS4 _Mtx_init_with_name where the name only names the pthread object. _Thrd_id keeps throwing: no public source settles the PS5 return form. All recorded in TechnicalDebt.
• guest_ampr_threads gains a waiter woken by signal on a named condition, two waiters where one signal wakes exactly one, signal with no waiters, a null name, a plain _Cnd_init handle, interop with libc _Cnd_broadcast/_Cnd_destroy, and the error cases.

Tested on Windows 11 Pro 26220, WinLibs MinGW-w64 GCC 15.2.0 r7, Ninja, Release, BUILD_TESTING=ON, full build including libs: guest_ampr_threads and guest_libc_threads 3/3; ctest -j8 -E "^agc_driver_|^video_out_flip" 507/507 (with the #3329 test fix applied locally so the tree builds); with main's Export.cpp the extended Ampr test fails; tools/check_conventions.py --base main and git diff --check pass. Not run on Linux. The "wakes exactly one" check uses a 50 ms window and would misfire only on a spurious host wakeup.

Written with Claude Code assistance (implementation by a subagent, reviewed and validated in this session).