Consolidate focus states into the focus-ring() mixin (#2972)

Step 1 of #2972, output-neutral. `focus-ring()` is now the one
implementation — `outline` + `box-shadow: $focus-ring-box-shadow`, with
`$shadow` (validation colour, range extra layer) and `$outline` (skip the
reset on sibling / `:focus-within` selectors) parameters. Every direct
`box-shadow: $*-focus-box-shadow` consumer calls it; the 14
`$*-focus-box-shadow` variables stay as `!default` aliases.

Compiled CSS byte-identical to `v2-dev`; html-diff, test:scss and lint green.

.changeset/focus-ring-mixin.md

@@ -0,0 +1,5 @@
+---
+"@tabler/core": patch
+---
+
+Updated `focus-ring()` to be the single focus-indicator mixin; the `$*-focus-box-shadow` variables stay as aliases.

BOOTSTRAP-V6-MIGRATION.md

@@ -253,6 +253,12 @@ of `color-scheme`, the token-dump verification harness) but not its code.
 - Decide on `$spacers`: adopting the v6 0–9 scale silently changes every `.p-3` / `.m-4` in every
   Tabler template. Recommendation: keep the v5 scale, add finer steps at new keys.
 - Replace all `$*-focus-box-shadow` with one `focus-ring()` mixin over `--focus-ring*` tokens.
+  Step 1 done (#2972): `focus-ring()` reads `$focus-ring-box-shadow` and every focus consumer calls
+  it; the `$*-focus-box-shadow` variables stay as `!default` aliases; CSS byte-identical. Tabler
+  keeps the two-layer `box-shadow` look, not v6's `outline`, and the `forced-colors` fallback in
+  `layout/_accessibility.scss` stays. `.form-select` is still a single-layer exception. Step 2
+  (emit `--focus-ring-box-shadow` on `:root`, fix the `.focus-ring` helper, decide the
+  `.form-select` ring) is a follow-up.
 - Consider the layered shadow scale (`.shadow-xs`…`.shadow-xl`) and `.border-keyline`.
 - Skip the `.fs-*` renaming — it inverts the scale direction and breaks every page.
 

core/scss/tests/_mixins.test.scss

@@ -149,6 +149,21 @@
       }
     }
   }
+
+  @include it('passes $shadow through and skips the outline reset with $outline: null') {
+    @include assert() {
+      @include output() {
+        .t {
+          @include focus-ring($shadow: 0 0 0 1px red, $outline: null);
+        }
+      }
+      @include expect() {
+        .t {
+          box-shadow: 0 0 0 1px red;
+        }
+      }
+    }
+  }
 }
 
 @include describe('media-print mixin') {