undefined assignment

TODO.md

@@ -166,6 +166,23 @@
     no hand-writable spelling and are emitted as `# unsupported in bindings:` comments
     (functions that reference an un-spellable union therefore keep a dangling reference)
 
+12. `undefined` as inspired by zig (implemented):
+  - allow mutable local declarations with `undefined`
+    - undefined values are assigned a poison value (0xaa...)
+    - allows for something like:
+      ```
+      a int = undefined
+      if (condition) {
+          a = 42
+      } else {
+          a = -2
+      }
+      ```
+  - disallow: `b :: undefined` since assigning undefined to something that can't change defeats the purpose
+  - disallow assigning `undefined` after declaration; use optionals and `none` for values that intentionally move back to an empty state
+
+13. broaden type inference from surrounding context
+
 ## A word on multi-unwrap
 
 Unwrap multiple optionals with `and`. This **short-circuits**: if the first optional is none, subsequent expressions are not evaluated.