extern: a unit parameter contributes no argument, wherever it sits (#123)

[simontreanor]

#123's table showed three shapes with three meanings: a nullary extern called with zero arguments (right), a leading unit splitting the call (unit -> int -> int = abs emitted abs()(-5) — the zero-argument rule fired and the remaining arguments applied to its result), and a mid-list unit travelling as None (max(3, None, 5)). This lands the issue's requested rule: a unit parameter contributes no Python argument, wherever it sits, which subsumes the nullary case instead of special-casing it.

• extern_unit_slots replaces nullary_externs: the positions of plain unit parameters along the curried list (distinct from thunk slots, which are function-typed unit -> a).
• A new build_call_units handles every arity: full application emits one call with the units dropped (abs(-5), max(3, 5)); over-application folds extras as everywhere else; partial application closes over a lambda — functools.partial cannot drop a unit that has not arrived, the lambda accepts and ignores it (pick 4 → lambda _pf_k0, _pf_k1: max(4, _pf_k1)). A dropped argument that could have effects still runs, as its own statement, with neighbours hoisted to keep left-to-right order.
• Bare references and project-mode bindings get unit_dropping_lambda (the all-unit case keeps today's lambda *_: shape); receiver methods, property folds and receiver_lambda drop units the same way; the unit block now also thunk-wraps, which the old nullary path never did.
• The parser's ...-slot arithmetic counts only non-unit arguments, so the extern the finding actually wanted — unit -> (unit ->{io} unit) -> Thread = threading.Thread(daemon = true, target = ...) — now parses, and emits threading.Thread(daemon=True, target=lambda: f(None)) in one call (verified end to end with a running thread). An all-unit type still rejects a slot with the existing message.

The mid-list None → dropped change is breaking in principle; nothing in the repo, lessons or tests relied on it, and the issue makes the design argument (int -> unit -> int -> int most plausibly meant max(3, 5)).

Docs: DESIGN.md §6 states the one rule and the amended slot arithmetic; ROADMAP.md item 24. Also carries the VS Code Marketplace ledger line to 0.8.0 (uploaded 2026-08-30), per the one-ledger-PR rule.

Closes #123