io interface (first pass)

TODO.md

@@ -807,7 +807,14 @@
     ordinary float `/` remains the unchecked IEEE infinity/NaN escape hatch
   - migrated `std/mem`, `std/arraylist`, and the compound-assignment example to `div_trunc`
 
-33. design io interface
+33. explicit I/O provider (implemented)
+  - `main` may take one canonical `@std/io Io`; parameterless entry points remain valid
+  - the compiler supplies a file-hidden macOS provider through an external no-argument C wrapper
+  - readers and writers pair an explicit provider with `stdin`, `stdout`, or `stderr`
+  - `read` and `write` validate provider counts; `write_all` handles partial writes and no progress
+  - the system provider uses unbuffered libc `read`/`write`, retries interruption, and allocates nothing
+
+34. re-exports so `std` can re-export e.g. `ArrayList(T)`, so users can do `std.ArrayList(i32)` instead of `std.arraylist.ArrayList(i32)`, while still using `std.arraylist.append(&values, 420)`. thinking this would be nice ergonomically.
 
 ## A word on unchecked casts