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>

docs/language/annotations/annot-system.md

@@ -0,0 +1,24 @@
+---
+id: annot-system
+name: @System
+category: annotations
+kind: annotation
+tokens: @System
+sig: "@System(Phase) function name() { … }"
+tip: Register a prebuilt binary module's system with the host at load, per phase.
+order: 62
+---
+
+<code>@System(Phase)</code> marks a function in a prebuilt binary module (one compiled with <code>--emit-module</code>) as a runtime-registered system. The module carries a load-time constructor that hands each such function to the host through <code>ludic_register_system</code>, and the host's frame loop calls it every <code>Phase</code> after its own handlers — the runtime, escape-hatch counterpart of the compile-time <code>@EngineSystem</code>. The compiler supplies the function's address (Ludic source cannot take one). This is how a closed-source or other-language module contributes systems over the reflection C-ABI; source packages that compile into the consumer should prefer <code>@EngineSystem</code>. Phases are <code>Input</code>, <code>FixedUpdate</code>, <code>Update</code>, <code>LateUpdate</code> and <code>Render</code>.
+
+```ludic
+program ManaModule {
+  function module_init() -> void { world_register_prop("Mana", 2) }   # register a component at load
+
+  @System(Update) function mana_regen() -> void {                     # runs every Update in the host
+    let M = world_prop_id("Mana")
+    let e = world_query_next(M, 0)
+    if e >= 0 { world_set(e, M, 0, world_get(e, M, 0) + 1) }
+  }
+}
+```