Full common split: composite tsconfigs and target-* modules

- Add composite TypeScript projects for common/primitives and common/runtime
- Decouple primitives from runtime (buffer service interfaces, UnicodeProperties, CellTypes, etc.)
- Move Types.ts to runtime; re-export primitive types from runtime/Types
- Rename src/browser -> src/target-browser, src/headless -> src/target-headless
- Update import paths (target-browser/*, target-headless/*) across repo
- Fix esbuild outbase so common/* and target-* resolve under out-esbuild/
- Point unicode addons at primitives for UnicodeTypes/UnicodeProperties
- Update ARCHITECTURE.md and AGENTS.md

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

test/playwright/tsconfig.json

@@ -18,8 +18,8 @@
     "declaration": true,
     "moduleResolution": "nodenext",
     "paths": {
-      "browser/*": [
-        "../../src/browser/*"
+      "target-browser/*": [
+        "../../src/target-browser/*"
       ],
       "common/*": [
         "../../src/common/primitives/*",
@@ -37,7 +37,7 @@
   ],
   "references": [
     {
-      "path": "../../src/browser"
+      "path": "../../src/target-browser"
     },
     {
       "path": "../../src/common"