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,

src/lsp/server.zig

@@ -271,7 +271,7 @@ pub const Server = struct {
         if (findImportPathAtOffset(doc.source, offset)) |import_path| {
             const base_dir = sx.imports.dirName(file_path);
             const rp: ?[]const u8 = if (self.root_path.len > 0) self.root_path else null;
-            const resolved = try sx.imports.resolveImportPath(self.allocator, self.io, base_dir, import_path, rp);
+            const resolved = try sx.imports.resolveImportPath(self.allocator, self.io, base_dir, import_path, rp, &.{});
 
             // For directory imports, try to read as file first
             if (std.Io.Dir.readFileAlloc(.cwd(), self.io, resolved, self.allocator, .limited(10 * 1024 * 1024))) |_| {