Split common into primitives and runtime layers

Reorganize src/common per issue #5963: move building blocks under
primitives/ and services/integration under runtime/. Preserve existing
common/* import paths via TypeScript path mapping. Add src/ARCHITECTURE.md
documenting modules and goals.

Fix two relative imports broken by the directory depth change.

Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>

addons/addon-progress/src/tsconfig.json

@@ -16,6 +16,8 @@
     ],
     "paths": {
       "common/*": [
+        "../../../src/common/primitives/*",
+        "../../../src/common/runtime/*",
         "../../../src/common/*"
       ],
       "browser/*": [