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

src/lsp/document.zig

@@ -184,7 +184,7 @@ pub const DocumentStore = struct {
             for (root.data.root.decls) |decl| {
                 if (decl.data != .import_decl) continue;
                 const imp = decl.data.import_decl;
-                const resolved_path = try sx.imports.resolveImportPath(self.allocator, self.io, base_dir, imp.path, self.rootPathOpt());
+                const resolved_path = try sx.imports.resolveImportPath(self.allocator, self.io, base_dir, imp.path, self.rootPathOpt(), &.{});
                 try import_list.append(self.allocator, .{
                     .ns = imp.name,
                     .path = resolved_path,