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/typespecs.md

@@ -2,6 +2,19 @@
 
 Patterns extracted from the Elixir standard library source code.
 
+## Contents
+
+1. [Public Type with @typedoc](#1-public-type-with-typedoc)
+2. [Private Types with @typep](#2-private-types-with-typep)
+3. [@opaque Types (Protocol t())](#3-opaque-types-protocol-t)
+4. [Union Types in @spec Return Values](#4-union-types-in-spec-return-values)
+5. [`when` Constraints in Specs](#5-when-constraints-in-specs)
+6. [Map Types with required/optional Keys](#6-map-types-with-requiredoptional-keys)
+7. [Keyword List Types for Options](#7-keyword-list-types-for-options)
+8. [Parameterized Types (t/1)](#8-parameterized-types-t1)
+9. [Named Parameters in Specs (:: annotation)](#9-named-parameters-in-specs--annotation)
+10. [@typedoc since: Annotation](#10-typedoc-since-annotation)
+
 ---
 
 ## 1. Public Type with @typedoc
@@ -798,4 +811,17 @@ end
 
 **Why:** `since:` annotations are for library consumers checking compatibility across versions. Application code doesn't have "consumers" checking which version introduced a type — it's all deployed together.
 
+## Decision Tree
+
+- If you are defining a public `@type` that appears in any `@spec` or callback → [Public Type with @typedoc](#1-public-type-with-typedoc)
+- If a type is used only internally for recursion or DRYing up repeated expressions → [Private Types with @typep](#2-private-types-with-typep)
+- If you want to hide internal representation and force consumers to use accessor functions → [@opaque Types](#3-opaque-types-protocol-t)
+- If a function can return multiple distinct shapes (tagged tuples, atoms) → [Union Types in @spec Return Values](#4-union-types-in-spec-return-values)
+- If the return type depends on the input type (generic/polymorphic function) → [`when` Constraints in Specs](#5-when-constraints-in-specs)
+- If you accept a map with a mix of mandatory and optional keys → [Map Types with required/optional Keys](#6-map-types-with-requiredoptional-keys)
+- If a function accepts a keyword list of options and you want to document valid keys → [Keyword List Types for Options](#7-keyword-list-types-for-options)
+- If you define a container type and want specs to express what element type is inside → [Parameterized Types (t/1)](#8-parameterized-types-t1)
+- If a parameter's type alone does not convey its purpose → [Named Parameters in Specs](#9-named-parameters-in-specs--annotation)
+- If you are adding a new public type to an existing library post-1.0 → [@typedoc since: Annotation](#10-typedoc-since-annotation)
+
 <!-- PATTERN_COMPLETE -->