Speed up build ~40% and add snapshot test suite (v1.6.0)

Replace the @extend-based comma-compression pass with a precomputed
grouped-selector built during IR construction, cutting a full build from
~4.8s to ~2.8s with functionally equivalent output. Move per-utility
registration to a flat shallow-merged registry instead of deep-merging the
entire config in all 173 utility files.

Add a snapshot test harness (npm test / test:update) with fixtures, a CSS
normalizer for equivalence checks, README docs, and Cursor rules covering the
utility schema and engine/test workflow. Rebuild dist and bump to v1.6.0.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: jinsupark

.cursor/rules/uniform-engine.mdc

@@ -0,0 +1,35 @@
+---
+description: UniformCSS engine architecture and test workflow
+globs: uniform/_index.scss,uniform/core/**/*.scss
+alwaysApply: false
+---
+
+# Engine: config → IR → emitters
+
+`uniform/_index.scss` is a small compiler:
+
+1. Utility files register into `core.$utilities` (a flat registry).
+2. `tree-constructor` lowers config into an intermediate map of class entries
+   (`selector`, `grouped-selector`, `properties`, `category`, `parent`,
+   `pseudo`, `screen`).
+3. Emitters consume that IR for `css`, `json`, `headless`, and `apply()`.
+
+## Do not reintroduce @extend for comma-compression
+
+Compressed output uses the precomputed `grouped-selector` (base selector +
+all its pseudo selectors, comma-joined) built during construction. This
+replaced the old `@extend` pass and is ~40% faster. Emit one rule per base
+class; skip `category: pseudo` entries. Never add `@extend` back.
+
+## Always run snapshot tests after engine changes
+
+```sh
+npm test            # byte-exact diff of fixtures vs test/__snapshots__/
+npm run test:update # refresh snapshots after an INTENTIONAL output change
+```
+
+- For compressed output, byte order may legitimately change. Verify functional
+  equivalence with `node test/normalize.js <a.css> <b.css>` (expands comma
+  groups and compares selector→declaration sets) BEFORE updating snapshots.
+- `nocompress`, `json`, and `apply` fixtures must stay byte-identical unless
+  you intentionally changed those paths.

.cursor/rules/uniform-utilities.mdc

@@ -0,0 +1,41 @@
+---
+description: How to add or edit a UniformCSS utility
+globs: uniform/utilities/**/*.scss
+alwaysApply: false
+---
+
+# Utility files are declarative config, not CSS
+
+Each utility file defines a single utility as a config map and registers it
+into the shared registry. It must NOT emit any CSS itself — the engine in
+`uniform/_index.scss` reads the registry and generates everything.
+
+## Required shape
+
+```scss
+@use "uniform/core";
+@use "sass:map";
+
+$config: (
+  utilities: (
+    align-content: (
+      important: false,
+      shorthand: align-content,   // class prefix; null = use variant key alone
+      responsive: true,
+      responsive-pseudos: false,
+      extra-selector: null,
+      properties: (align-content),
+      static-properties: (),      // always-emitted props (e.g. CSS vars)
+      variants: ( center: center, start: flex-start ),
+      pseudos: (none)             // or e.g. (hover, group-hover, focus)
+    )
+  )
+);
+
+// Always register via this exact line — shallow merge into the registry.
+core.$utilities: map.merge(core.$utilities, map.get($config, utilities));
+```
+
+- Do NOT use `map.deep-merge(core.$all-config, $config)` (the old, quadratic pattern).
+- Utility names are unique top-level keys, so a shallow `map.merge` is correct.
+- After adding a utility, also `@use` it in `uniform/_index.scss`.

README.md

@@ -132,6 +132,32 @@ Add Uniform as a Sass module to your `main.scss` project. Follow these steps to
    );
    ```
 
+## Development & Testing
+
+Uniform compiles a config map into an intermediate representation and then emits
+CSS, JSON, or headless output. To guard that pipeline, the repo ships a snapshot
+test harness that compiles a set of fixtures and diffs them against committed
+baselines.
+
+```sh
+# Compile fixtures and diff against test/__snapshots__/
+npm test
+
+# Refresh baselines after an intentional output change
+npm run test:update
+```
+
+Fixtures live in `test/fixtures/` and cover the default build, the
+non-compressed path, JSON output, a heavily customized config, and the
+`apply()` mixin. Because comma-compressed output can change selector order
+without changing meaning, `test/normalize.js` expands comma groups and compares
+`selector → declaration` sets so you can confirm two stylesheets are
+functionally equivalent before updating a snapshot:
+
+```sh
+node test/normalize.js test/__snapshots__/default.css test/__out__/default.css
+```
+
 ## Community
 
 If you're ever stuck, need help, or wish to have a general discussion about this project, please get involved with the following community channels.