docs: record why {} is a set and {:} is a map

docs/design/collections.md

@@ -11,6 +11,29 @@ arbitrary nesting (e.g. `list<map<string, list<int>>>`).
 | `map<K,V>` | `BTreeMap<Value, Value>` | key/value pairs, sorted keys |
 | `set<T>` | `BTreeSet<Value>` | unique elements, membership |
 
+## Empty literals: `{}` is a set, `{:}` is a map
+
+`map` and `set` share brace syntax, so an empty `{}` is ambiguous on its face.
+Mux resolves this in the grammar rather than the type system: **`{}` is always
+the empty set, and the empty map is spelled `{:}`.**
+
+The alternative - inferring which one `{}` meant from the surrounding expected
+type - is what the compiler used to do, via a third `Type::EmptySetOrMap` that
+every stage had to carry and a span-keyed override map that rewrote the type
+after the fact. It worked, but the ambiguity leaked into semantics and codegen,
+and an empty literal with no expected type to resolve against had no answer.
+`{:}` removes the ambiguity at the source, so `EmptySetOrMap` and the override
+machinery are gone (mux-compiler#266).
+
+Consequences worth knowing:
+
+- `{}` in a map-typed position is a compile error, not an inference. Both
+  directions of the mix-up get a targeted diagnostic naming the other spelling.
+- Empty literals still need an explicit type - `{:}` alone cannot infer `K`/`V`,
+  the same way `[]` cannot infer its element type.
+- Nesting follows the same rule per position: `map<int, set<int>> x = {1: {}}`
+  is a map of sets, while `map<int, map<int, int>> y = {1: {:}}` is a map of maps.
+
 ## Why BTree, not Hash
 
 `map` and `set` use the B-tree variants rather than hash maps for:

llms.txt

@@ -15,7 +15,7 @@ repo owns the facts that span more than one repo.
 - [Monomorphization](https://github.com/muxlang/mux-context/blob/main/docs/design/monomorphization.md): compile-time generics
 - [Object system](https://github.com/muxlang/mux-context/blob/main/docs/design/object-system.md): ObjectRef, type registry, static interface dispatch
 - [Operators](https://github.com/muxlang/mux-context/blob/main/docs/design/operators.md): short-circuit lowering, operator overloading
-- [Collections](https://github.com/muxlang/mux-context/blob/main/docs/design/collections.md): list/map/set, BTree rationale, nesting
+- [Collections](https://github.com/muxlang/mux-context/blob/main/docs/design/collections.md): list/map/set, BTree rationale, nesting, empty literals (`{}` is a set, `{:}` is a map)
 - [Error handling](https://github.com/muxlang/mux-context/blob/main/docs/design/error-handling.md): result/optional layout
 - [Runtime panics](https://github.com/muxlang/mux-context/blob/main/docs/design/panics.md): process-terminating failures, the panic format, why no baked source snippet
 - [Modules](https://github.com/muxlang/mux-context/blob/main/docs/design/modules.md): import resolution, name mangling, module init