Replace monolithic Master Reference with focused Sources page

The 1711-line master reference was 100% duplicated across the existing
37 Starlight pages. Trimmed to bibliography only (Ghidra port numbers,
driver source paths, analysis reports, community links).

Salvaged FX2 USB controller registers (SETUPDAT, CPUCS, EP0BUF, etc.)
into bcm4500/register-map.mdx before removal. Sidebar label updated
from "Master Reference" to "Sources".

site/astro.config.mjs

@@ -122,7 +122,7 @@ export default defineConfig({
         {
           label: 'Reference',
           items: [
-            { label: 'Master Reference', slug: 'reference/master-reference' },
+            { label: 'Sources', slug: 'reference/master-reference' },
           ],
         },
       ],