feat(libc): implement the 16-byte __sync builtins and __atomic_is_lock_free

[hypermoist]

What

Implements the libatomic-style exports of the system libc.prx that AnyPS5 did not have, in a new src/AtomicBuiltins.cpp:

• __sync_fetch_and_add_16, __sync_fetch_and_sub_16, __sync_fetch_and_and_16, __sync_fetch_and_or_16, __sync_fetch_and_xor_16, __sync_lock_test_and_set_16: a lock cmpxchg16b loop returning the previous value, as on the console (GNU inline asm, so no -mcx16/libatomic is needed on either host)
• __atomic_is_lock_free(size, ptr): sizes 0 and 1 are lock-free, 2/4/8/16 when ptr is aligned to the size, anything else is not (the console's jump table)
• __atomic_load, __atomic_store, __atomic_exchange, __atomic_compare_exchange: in the libc.prx bundled with PPSA08427 each of these is a single ud2, so a call traps on the console; they throw here, recorded in docs/dev/debt/libc.md

Tested

Linux (CachyOS, GCC Release build, plus a clang -Wall -Wextra syntax check): new guest_atomic_builtins test (each operation, the lock-free table, the traps, and two threads racing __sync_fetch_and_add_16 across the 64-bit carry) passes. tools/check_conventions.py passes; tools/pr_overlap.py --branch reports no overlap.

Differential test against the console: the PPSA08427 libc.prx was mapped into a Linux process and __atomic_is_lock_free (sizes 0-39, 64 alignments) and each 16-byte operation (100,000 random operand pairs) were run side by side with this implementation; returned values and memory agree everywhere.

Checklist

• Based on current main; no other open PR implements the same functions
• One topic per PR; follow-ups go in a new PR
• No comments in code except technical debt
• Unimplemented paths throw (NotImplemented_nid_no_patch); silent stubs are listed under Silent stubs in their library's technical debt file
• No notes, investigation .md files or images added to the repository (attach them to this PR)
• New third-party code is a submodule built from source
• Generic behaviour, not specific to one title
• Depends on: none
• Unblocks: none
• GPUs tested: n/a
• AI-assisted: yes

🤖 Generated with Claude Code

https://claude.ai/code/session_01Q8tinbP1MUpUaDsbNWnGnu

[github-actions]

Progress report (b3f9f89 → 6d2708a)

📈 System libraries: 88.75% (+0.04%, +11 implemented functions)

✅ 11 implemented

Library	Name
libc	__atomic_compare_exchange_nid_postfix
libc	__atomic_exchange_nid_postfix
libc	__atomic_is_lock_free_nid_postfix
libc	__atomic_load_nid_postfix
libc	__atomic_store_nid_postfix
libc	__sync_fetch_and_add_16_nid_postfix
libc	__sync_fetch_and_and_16_nid_postfix
libc	__sync_fetch_and_or_16_nid_postfix
libc	__sync_fetch_and_sub_16_nid_postfix
libc	__sync_fetch_and_xor_16_nid_postfix
libc	__sync_lock_test_and_set_16_nid_postfix

[github-actions]

Some comparisons could not be completed because a pull request conflicts with the base branch. Review these shared files manually before concluding that there is no overlap.

Pull request	Shared files not compared
PR 3987	docs/dev/debt/libc.md
PR 5021	core/libs/prx/libc/CMakeLists.txt docs/dev/debt/libc.md
PR 5303	docs/dev/debt/libc.md
PR 6044	core/libs/prx/libc/CMakeLists.txt
PR 6226	docs/dev/debt/libc.md
PR 6565	docs/dev/debt/libc.md
PR 7039	docs/dev/debt/libc.md
PR 7081	core/libs/prx/libc/CMakeLists.txt docs/dev/debt/libc.md
PR 7084	core/libs/prx/libc/CMakeLists.txt
PR 7201	core/libs/prx/libc/CMakeLists.txt
PR 7332	core/libs/prx/libc/CMakeLists.txt docs/dev/debt/libc.md