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/compiler.sx

@@ -0,0 +1,15 @@
+OperatingSystem :: enum { macos; linux; windows; wasm; ios; unknown; }
+Architecture :: enum { aarch64; x86_64; wasm32; wasm64; unknown; }
+
+OS : OperatingSystem = .unknown;
+ARCH : Architecture = .unknown;
+POINTER_SIZE : s64 = 8;
+
+BuildOptions :: struct {
+    add_link_flag :: (self: BuildOptions, flag: [:0]u8) #compiler;
+    add_framework :: (self: BuildOptions, name: [:0]u8) #compiler;
+    set_output_path :: (self: BuildOptions, path: [:0]u8) #compiler;
+    set_wasm_shell :: (self: BuildOptions, path: [:0]u8) #compiler;
+}
+
+build_options :: () -> BuildOptions #compiler;