📝 Document why open universes are rejected

Add a README Caveats subsection with user-facing guidance for the two
open-universe shapes: parse external input at the boundary down to a
finite union, or use a runtime Map registry for extensible domains.
Point the finite-universe bullet at it, and point the rejected
fix/open-universe-* entry in development/library.md back at the new
guidance, so rule and decision cross-reference without duplicating each
other.

README.md

@@ -40,6 +40,19 @@ Yet to be implemented
 - **`NaN` and `-0` cannot be matched specifically.** They have no literal type,
   so both stay part of `number`.
 
+### Why open universes are rejected
+
+An open universe — one carrying a broad member, as in
+`type Units = "s" | "ms" | "min" | (string & {})` — is not a dispatch concern.
+If the values arrive from outside the program, parse them at the boundary down
+to a finite union and match the narrowed result; the openness never reaches the
+matcher. If the domain is genuinely extensible, the right shape is a runtime
+`Map` of handlers, where "no handler" is a lookup, not a pattern. Either way an
+open matcher would abandon the one guarantee this library exists to give —
+provable exhaustiveness — to automate what a `switch` and a default arm already
+cover. The type-level cost of supporting open universes is recorded in
+[development/library.md](./development/library.md#supported-universes).
+
 ## License
 
 MIT © 2025 tmu. See [LICENSE](./LICENSE).

development/library.md

@@ -167,7 +167,9 @@ Extract<T, Stringified<T>>` catches numeric collisions too.
   non-self-referential parameter type (`{ [K in keyof H]: … R … }` gives
   `unknown`); an F-bounded guard referencing `keyof Handled` in `Handled`'s own
   constraint sees the constraint, not the map; all handlers share one `R` (only
-  the fallback widens it); `IsLiteral` is the finite-literal predicate.
+  the fallback widens it); `IsLiteral` is the finite-literal predicate. The
+  user-facing consequence — open universes are a parsing or registry concern,
+  not a dispatch one — is guidance in README § Why open universes are rejected.
 - **`Member<T, K>` without the gate:** sound, but a colliding handler gets a
   union and `1 | "1"` stays one runtime key.
 - **A round-trip injectivity gate** (`IsEqual<T, PatternParam<PatternKey<T>>>`):