docs: backfill TOC + decision trees, fix review findings

- Add ## Contents and ## Decision Tree to all 10 existing pattern files
- Fix embed_as/1 semantics inversion in types.md (:self → :dump)
- Fix fabricated __meta__.changes reference in changesets.md
- Fix default primary key type (:integer → :id) in schemas.md
- Combine @impl subsections into single "Minimal Callback Annotation"

patterns/changesets.md

@@ -328,17 +328,16 @@ end
 **Anti-pattern:** Reimplementing the "has change?" check inside the validator:
 
 ```elixir
-# BAD — redundant check; validate_change already handles this
+# BAD — redundant check; validate_change already skips if :sku is unchanged
 def changeset(item, params) do
-  item
-  |> cast(params, [:sku])
-  |> validate_change(:sku, fn :sku, value ->
-    if Map.has_key?(item.__meta__.changes, :sku) do
-      if valid_sku?(value), do: [], else: [{:sku, "invalid format"}]
-    else
-      []
-    end
-  end)
+  changeset = cast(item, params, [:sku])
+
+  if Map.has_key?(changeset.changes, :sku) do
+    value = get_change(changeset, :sku)
+    if valid_sku?(value), do: changeset, else: add_error(changeset, :sku, "invalid format")
+  else
+    changeset
+  end
 end
 ```