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

build.zig

@@ -158,6 +158,16 @@ pub fn build(b: *std.Build) void {
 
     b.installArtifact(exe);
 
+    // Install the stdlib alongside the binary so `<prefix>/bin/sx` finds
+    // `<prefix>/library/modules/...` via the install-layout fallback in
+    // `src/imports.zig::discoverStdlibPaths`.
+    const install_library = b.addInstallDirectory(.{
+        .source_dir = b.path("library"),
+        .install_dir = .prefix,
+        .install_subdir = "library",
+    });
+    b.getInstallStep().dependOn(&install_library.step);
+
     const run_step = b.step("run", "Run the app");
     const run_cmd = b.addRunArtifact(exe);
     run_step.dependOn(&run_cmd.step);