docs(pkg): document @Namespace / @EngineSystem / @System + package hooks (#62)

Per-annotation pages for the three package-registration annotations (with
inventory entries), a "Package-declarable namespaces and engine systems" section
in docs/PACKAGES.md, and a changeset. All doc gates green (check-docs 675
fences, docs-check 726 symbols).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tools/docgen/inventory.json

@@ -21,7 +21,10 @@
     "annot-server",
     "annot-predicted",
     "annot-toserver",
-    "annot-toclients"
+    "annot-toclients",
+    "annot-system",
+    "annot-enginesystem",
+    "annot-namespace"
   ],
   "builtins": [
     "fn-print",