Add comprehensive TW test suite (627 tests), fix 4 bugs, add 9 features

Bugs fixed:
- line-through: check full token not just head after hyphen split
- gap-x-N/gap-y-N: compound 2-part spacing prefix handler in tw-layout
- Negative values (-mt-4): replace ":" with ":-" instead of no-op
- Class name doubling: chain replace calls instead of concatenating

New features in tw-process-token:
- !important modifier (!p-4 → padding:1rem !important)
- dark: variant (class-based, .dark ancestor selector)
- group-hover:/group-focus:/group-active: (parent state)
- peer-focus:/peer-hover:/peer-checked:/peer-disabled: (sibling state)
- @container queries (@md:flex → @container(min-width:448px))
- Colour opacity modifier (bg-sky-500/50 → hsl with alpha)
- Ring colours (ring-sky-500 → --tw-ring-color)
- Arbitrary values (w-[300px], grid-cols-[1fr_2fr], bg-[#ff0000])
- colour-with-alpha helper for HSL+alpha generation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

shared/sx/templates/tw-layout.sx

@@ -24,6 +24,13 @@
           ((tmpl (get tw-spacing-props head))
             (v (tw-spacing-value (first rest))))
           (if (nil? v) nil (tw-template tmpl v)))
+        (and
+          (= (len rest) 2)
+          (get tw-spacing-props (str head "-" (first rest))))
+        (let
+          ((tmpl (get tw-spacing-props (str head "-" (first rest))))
+            (v (tw-spacing-value (last rest))))
+          (if (nil? v) nil (tw-template tmpl v)))
         (and
           (= head "space")
           (= (len rest) 2)