✨ Match boolean, null and undefined

Extend the matcher universe beyond string | number so a union can carry
boolean, null and undefined members. true|false, null and undefined are
not property keys, so handler keys are their stringification while the
callback still receives the real member. Symbols are rejected (a brand
is compile-time only) and bigint is not a property key.

development/library.md

@@ -91,3 +91,36 @@ Each factory is two overloads whose order is load-bearing:
 - `Parameters<typeof factory>[0]` resolves only the **last** overload, so it is
   not a sound "rejected" oracle for a factory. Factory-negative tests use
   `@ts-expect-error` call sites (the test file only — the general ban stands).
+
+## Primitive universe
+
+#### Decision (2026-09)
+
+The universe (`Matchable`) is `string | number | boolean | null | undefined`,
+with `boolean` admitted as `true | false`. `symbol` and `bigint` are not.
+
+`boolean`/`null`/`undefined` are not property keys, so handler-map keys are a
+projection (`PatternKey`: each member stringified) and `PatternParam` inverts
+it, so callbacks receive the real member (`true`, not `"true"`). The popup
+offers `true`, `false`, `null`, `undefined` by name (verified over LSP).
+
+#### Why
+
+- Runtime dispatch is unchanged in effect: `handlers[true]` already coerces to
+  `"true"`. `dispatch` wraps the index in `String()` only because TypeScript
+  forbids indexing with `boolean`/`null`/`undefined` (`TS2538`).
+
+#### Rejected
+
+- **`symbol`.** A brand is a compile-time phantom — nothing to match at
+  runtime; the popup cannot offer symbol keys anyway.
+- **`bigint`.** Not a valid property key; a stringified key (`"1"`) collides
+  with numeric `1`.
+- **`NaN` / `-0`.** No literal type exists; they stay one `number`.
+
+#### Known issue
+
+- A universe mixing a member with its stringification (`1 | "1"`,
+  `true | "true"`, `null | "null"`) collapses to one handler key and routes
+  both members to it. Pre-existing for `1 | "1"`; now reachable for the new
+  members. Not guarded at the type level.