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>

test/benchmark/tsconfig.json

@@ -13,7 +13,11 @@
     "target": "es2021",
     "module": "nodenext",
     "paths": {
-      "common/*": [ "../../src/common/*" ],
+      "common/*": [
+        "../../src/common/primitives/*",
+        "../../src/common/runtime/*",
+        "../../src/common/*"
+      ],
       "browser/*": [ "../../src/browser/*" ],
       "Terminal": ["../../src/Terminal"]
     },

test/playwright/tsconfig.json

@@ -22,6 +22,8 @@
         "../../src/browser/*"
       ],
       "common/*": [
+        "../../src/common/primitives/*",
+        "../../src/common/runtime/*",
         "../../src/common/*"
       ]
     },