📝 Document the finite-only universe decision

Record why open universes and value/stringification collisions are
rejected, the `Member<T, K>` inversion, and the findings from the
rejected open-universe attempt so they are not re-run.

README.md

@@ -25,10 +25,15 @@ Yet to be implemented
 
 ## Caveats
 
-- **A value and its stringification collide.** Object keys stringify, so a
-  universe that mixes a member with the string it stringifies to — `1 | "1"`,
-  `true | "true"`, `null | "null"` — collapses to a single handler key and both
-  members are routed to it. Use one form or the other.
+- **Only finite universes are supported.** The factory must be given a finite
+  union of literals; `string`, `number` and template literals are rejected. This
+  is what lets the exhaustive overload be proven, so the runtime `dispatch`
+  throw stays unreachable through the typed API.
+- **A value and its stringification must not both be present.** Object keys
+  stringify, so a universe containing both a member and the string it
+  stringifies to — `1 | "1"`, `true | "true"`, `null | "null"` — is rejected at
+  the factory. Either form alone is fine, and one value's string form may
+  coexist with a _different_ value's bare form (`"true" | false`).
 - **`symbol` and `bigint` are not supported.** A `symbol` brand is a
   compile-time phantom with nothing to match at runtime, and a `bigint` is not a
   valid property key; neither satisfies the matcher's universe constraint.