✨ 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.

backlog.tasks

@@ -52,8 +52,9 @@ Bugs:
     → `handleExit` returns `io.EOF`, cancelling the background context while `Session.updateWatches` is still in flight; the bare error is flushed to stderr and the server exits 1
     → close stdin after `shutdown` instead of sending `exit`; the server exits cleanly (code 0, no output), kill kept as a fallback
 Enhancements:
-☐ Allow boolean literals in primitive union patterns (e.g. `true: () => "yes"`) @medium
-☐ Are there other primitive types that should be supported in union patterns? (e.g. `bigint`, `symbol`) @medium
+✔ Allow boolean literals in primitive union patterns (e.g. `true: () => "yes"`) @medium @done
+    ✔ Are there other primitive types that should be supported in union patterns? (e.g. `bigint`, `symbol`) @medium @done
+        → added boolean, null and undefined; rejected `symbol` (compile-time brand, nothing at runtime) and `bigint` (not a property key)
 
 Documentation:
 ☐ Bring README.md back to its previous form — synopsis and examples restored, in the correct place