add float constraint type

TODO.md

@@ -181,7 +181,29 @@
   - 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
+13. introduce a `float` type constraint (similar to `int`) (implemented)
+  - resolves a local binding to any float scalar (`f32`/`f64`) via static analysis; widens
+    `f32` -> `f64` across assignments, mirroring how `int` picks the smallest fitting integer
+  - on a local declaration, integer literals satisfy `float` and default to `f64`
+    (`pi float = 3` is `3.0`); a runtime integer (`x float = some_i32`) stays a
+    `cannot implicitly convert` error
+  - a local initializer whose numeric family doesn't satisfy the constraint now errors for
+    both `int` and `float` instead of silently taking the initializer's natural type
+  - as with `int`, a constraint in a param/result position is a generic passthrough (it
+    forwards the inferred type unchanged, e.g. an identity `func(v int) int` over a range),
+    so the literal-as-float and family checks apply to local bindings, not passthroughs
+
+14. add slice-by-range
+  - allow the use of a range in slice expressions:
+    ```
+    excl_range range :: 0..10
+    some_arr[excl_range] # slice by named exclusive range
+  
+    incl_range range :: 0..=10
+    some_arr[incl_range] # slice by named inclusive range
+    ```
+
+15. broaden type inference from surrounding context
 
 ## A word on multi-unwrap