roadmap: park fieldtest F3 (zero-arg (app f) rejected at parse) as P2 todo

Keeps the mut-local fieldtest's last unrouted finding from rotting.
F1 is owned by the Iteration-discipline P1 milestone; F4 is absorbed
by its spec; F3 is an independent surface decision, parked here.

docs/roadmap.md

@@ -305,6 +305,14 @@ clean. Pick the next milestone from P2.)_
   type defs as candidates the way `bare-cross-module-type-ref`
   lists candidates from imports.
   - context: fieldtest 2026-05-11 — `examples/ct_3*.ail` exhibits both branches with identical-shape diagnostics.
+- [ ] **\[todo\]** Zero-arg `(app f)` rejected at parse — the Form-A
+  parser refuses an application with an empty argument list, so a
+  nullary call has no surface form. Surfaced by the mut-local
+  fieldtest (F3). Decide: accept `(app f)` as the nullary-call
+  surface, or ratify in DESIGN.md that nullary functions are
+  expressed differently (and say how). Not a blocker; no current
+  corpus program needs it, but an LLM author reaches for it.
+  - context: `docs/specs/2026-05-15-fieldtest-mut-local.md` finding F3.
 - [ ] **\[todo\]** Workspace search beyond entry-module's directory —
   `load_workspace` only finds sibling `.ail.json` files in the same
   directory as the entry module, so any consumer of prelude/std in a