stdlib: relocate modules under library/

- examples/modules/ -> library/modules/ (top-level, no more
  symlink hacks in consumer projects)
- compiler discovers stdlib via _NSGetExecutablePath / readlink
  /proc/self/exe; searches dev layout (../../library), install
  layout (../library), and alongside-binary fallback
- SX_STDLIB_PATH env var overrides for tests / dev convenience
- SX_DEBUG_STDLIB env var dumps the discovery results
- build.zig installs library/ alongside the binary
- Compilation gains stdlib_paths field threaded through resolveImports
- 50 tests pass; consumer projects can now build from any cwd

library/modules/socket.sx

@@ -0,0 +1,33 @@
+// POSIX socket module (macOS only)
+// sockaddr_in layout and constants are platform-specific.
+
+libc :: #library "c";
+
+// POSIX socket API
+socket     :: (domain: s32, kind: s32, protocol: s32) -> s32 #foreign libc;
+setsockopt :: (fd: s32, level: s32, optname: s32, optval: *s32, optlen: u32) -> s32 #foreign libc;
+bind       :: (fd: s32, addr: *SockAddr, addrlen: u32) -> s32 #foreign libc;
+listen     :: (fd: s32, backlog: s32) -> s32 #foreign libc;
+accept     :: (fd: s32, addr: *SockAddr, addrlen: *u32) -> s32 #foreign libc;
+read       :: (fd: s32, buf: [*]u8, count: usize) -> isize #foreign libc;
+write      :: (fd: s32, buf: [*]u8, count: usize) -> isize #foreign libc;
+close      :: (fd: s32) -> s32 #foreign libc;
+
+// Constants (macOS)
+AF_INET      :s32: 2;
+SOCK_STREAM  :s32: 1;
+SOL_SOCKET   :s32: 0xFFFF;
+SO_REUSEADDR :s32: 0x4;
+
+// macOS sockaddr_in (16 bytes, has sin_len field)
+SockAddr :: struct {
+    sin_len:    u8;
+    sin_family: u8;
+    sin_port:   u16;
+    sin_addr:   u32 = 0;
+    sin_zero:   u64 = 0;
+}
+
+htons :: (port: s64) -> u16 {
+    cast(u16) (((port & 0xFF) << 8) | ((port >> 8) & 0xFF));
+}