feat(composition): expose get_container for inventory readback

Add composition.get_container(entity) -> table or nil. Returns the
container block from the entity's template (e.g. {kind="list"}), or
nil if the template declared no container block. Loud-Error if the
entity was not created by composition.

The internal storage (tpl.container) was already present since v0.2.0
(stashed on the template record with a comment "B.2 reads this") but
had no public getter. This fills the oversight without any semantic
change to v0.2.0.

init.lua

@@ -427,4 +427,21 @@ function M.list_templates()
     return out
 end
 
+-- Returns the container block from the entity's template, or nil if the
+-- template declared no container block. Loud-Error if entity was not
+-- created by composition.
+function M.get_container(target_entity)
+    local reg_id = target_entity and target_entity:get_property("composition.reg_id")
+    if not reg_id or reg_id == 0 then
+        error("composition.get_container: entity was not created by composition")
+    end
+    local meta = entity_meta[reg_id]
+    if not meta then
+        error("composition.get_container: entity was not created by composition")
+    end
+    local tpl = templates[meta.template_id]
+    if not tpl then return nil end
+    return tpl.container  -- nil when not declared
+end
+
 return M