♻️ Share the Matchable universe between both matchers

development/library.md

@@ -97,9 +97,9 @@ Each factory is two overloads whose order is load-bearing:
 
 #### Decision (2026-09)
 
-`src/matcher-shared.ts` holds the six universe-agnostic pieces both matchers
-use: `UnaryFn`, `PatternReturns`, `RedundantFallback`, `HandlerMap`, and the
-`PatternKey` / `PatternParam` key projection.
+`src/matcher-shared.ts` holds the seven universe-agnostic pieces both matchers
+use: `UnaryFn`, `PatternReturns`, `RedundantFallback`, `HandlerMap`, the shared
+`Matchable` universe, and the `PatternKey` / `PatternParam` key projection.
 
 #### Why
 
@@ -107,6 +107,10 @@ use: `UnaryFn`, `PatternReturns`, `RedundantFallback`, `HandlerMap`, and the
   keeps the two matchers' message from drifting; the other pieces appear
   verbatim in both public signatures or are the same projection over each
   matcher's universe.
+- **`Matchable` is one definition, not two.** The primitive-union matcher's
+  universe and the tagged-union matcher's allowed `Tag` values are the same set,
+  so aliasing them keeps the two matchers from drifting apart on what they
+  accept (`symbol`/`bigint` rejected once).
 
 #### Rejected
 
@@ -114,8 +118,8 @@ use: `UnaryFn`, `PatternReturns`, `RedundantFallback`, `HandlerMap`, and the
   `Fallback` and `MustBePartial`.** Each is built from its own universe
   (`Tags`/`MapTaggedUnion` vs the primitive values); abstracting over the
   F-bounded `Handled` constraint that makes the remainder work risks the
-  contextual typing it exists to preserve. The `PatternKey` / `PatternParam`
-  projection is the one piece both universes genuinely share.
+  contextual typing it exists to preserve. `Matchable` and the `PatternKey` /
+  `PatternParam` projection are the pieces both universes genuinely share.
 
 ## Tagged-union matcher