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>

addons/addon-progress/src/tsconfig.json

@@ -20,8 +20,8 @@
         "../../../src/common/runtime/*",
         "../../../src/common/*"
       ],
-      "browser/*": [
-        "../../../src/browser/*"
+      "target-browser/*": [
+        "../../../src/target-browser/*"
       ],
       "@xterm/addon-progress": [
         "../typings/addon-progress.d.ts"
@@ -34,7 +34,7 @@
   ],
   "references": [
     {
-      "path": "../../../src/browser"
+      "path": "../../../src/target-browser"
     },
     {
       "path": "../../../src/common"