feat: add ButtonGroup, theme toggle, dark theme fixes, remove shift references

- Add ButtonGroup primitive: multi-select toggle with colored dot indicators
- Replace FilterPill status filters with ButtonGroup in TopBar and EventFeed
- Add light/dark mode toggle to TopBar (moon/sun icon)
- Fix dark theme: add --purple/--purple-bg tokens, replace all hardcoded
  #F3EEFA/#7C3AED with tokens, fix --amber-light text contrast in sidebar,
  brighten --sidebar-text/--sidebar-muted tokens, use color-mix for
  ProcessorTimeline bar fills
- Remove all "shift" references (presets, labels, badges)
- Shrink SegmentedTabs height to match search bar and ButtonGroup
- Update COMPONENT_GUIDE.md with new components and updated descriptions
- Add ButtonGroup demo to Inventory
- Add README.md with setup instructions and navigation guide

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

src/pages/Routes/Routes.module.css

@@ -333,8 +333,8 @@
 }
 
 .typeRouter {
-  background: #F3EEFA;
-  color: #7C3AED;
+  background: var(--purple-bg);
+  color: var(--purple);
 }
 
 .typeProcessor {

src/pages/Routes/Routes.tsx

@@ -59,7 +59,7 @@ function KpiHeader({ scopedMetrics }: { scopedMetrics: RouteMetricRow[] }) {
         <div className={styles.kpiLabel}>Total Throughput</div>
         <div className={styles.kpiValueRow}>
           <span className={`${styles.kpiValue} ${styles.kpiValueAmber}`}>{totalExchanges.toLocaleString()}</span>
-          <span className={styles.kpiUnit}>msg/shift</span>
+          <span className={styles.kpiUnit}>exchanges</span>
           <span className={`${styles.kpiTrend} ${styles.trendUpGood}`}>&#9650; +8%</span>
         </div>
         <div className={styles.kpiDetail}>
@@ -483,7 +483,7 @@ export function Routes() {
               <span className={styles.tableTitle}>Processor Performance</span>
               <div className={styles.tableRight}>
                 <span className={styles.tableMeta}>{processorMetrics.length} processors</span>
-                <Badge label="SHIFT" color="primary" />
+                <Badge label="LIVE" color="success" />
               </div>
             </div>
             <DataTable