iter nullary-app.1 (DONE 5/5): accept (app f) as canonical zero-arg call

Resolves Gitea #12 — the design fork from the 2026-05-20 /boss
session, surfaced independently by two prior fieldtests
(mut-local F3 + loop-recur `run_forever`).

Mechanics, layer by layer:

  Parser — `crates/ailang-surface/src/parse.rs:1322-1331` drops
    the 4-line `expected at least one argument` guard in
    `parse_app_body`. Grammar comments at file top change `term+`
    to `term*` on both `app-term` and `tail-app-term`; doc comment
    on `parse_app_body` changes `1+ args` to `0+ args`. Inline
    comment cites #12 and notes which layers below already accept
    empty args.

  Serde — `crates/ailang-core/src/ast.rs:419-425` annotates
    `Term::App.args` with `#[serde(default)]`, mirroring
    `Term::Ctor.args`'s *actual* attrs verbatim (no
    `skip_serializing_if`). Write behaviour unchanged — canonical
    JSON still emits `"args":[]` for nullary calls; only read
    behaviour gains tolerance for the `args` key being absent.
    Verified hash-impact-free at plan time:
    `grep -rn '"args":\[\]' examples/*.ail.json` returns zero
    matches, so no existing fixture's canonical bytes shift.

  Doc-honesty — `design/contracts/data-model.md`:
    (a) the `app` jsonc shape gains an explicit "args may be
        empty / read-tolerant" note;
    (b) the existing `ctor` comment claiming "args omitted when
        empty" was factually false (Ctor.args has no
        `skip_serializing_if`; a serde probe at plan time
        confirmed writes always emit `"args":[]`). Rewritten to
        describe the actual write/read asymmetry, with a
        cross-reference to the new `app` note.

  E2E pin — `crates/ail/tests/nullary_app_e2e.rs` +
    `examples/nullary_app_smoke.ail`. Defines `greet : fn()
    -> Unit !IO` and calls it as `(app greet)`; asserts stdout
    `"hello\n"`. RED→GREEN pin for the parser change AND the
    milestone-protecting E2E for nullary call surface going
    forward. Fixture literal is `"hello"` (no `\n`) because
    `io/print_str` lowers via `puts` which appends a newline —
    the plan body had a `"hello\n"` literal which would have
    yielded `"hello\n\n"`; the implementer caught and aligned to
    the canonical pattern in `examples/hello.ail` while writing
    the fixture, fix scoped to that single file.

Layers below parser untouched: typechecker's arity check at
`crates/ailang-check/src/lib.rs:3300` is `args.len() !=
params.len()` which is `0 != 0 → false` for nullary; codegen's
`args.iter().zip(sig.params.iter())` at
`crates/ailang-codegen/src/lib.rs:2410` is an empty loop;
LLVM `call @ail_<mod>_<name>()` with empty arglist is valid;
surface printer's arg-emit loop at
`crates/ailang-surface/src/print.rs:430-434` writes nothing for
empty args, producing exactly `(app f)`.

Verification: full workspace `cargo test --workspace --quiet`
green (0 failed across all crates); drift pins
`design_index_pin 5/5` + `design_schema_drift 8/8`; round-trip
`2/2`; new `nullary_app_e2e 1/1`. Stats file:
`bench/orchestrator-stats/2026-05-20-iter-nullary-app.1.json`.

closes #12

design/contracts/data-model.md

@@ -109,6 +109,9 @@ narrative — defaults, superclasses, diagnostics — lives in
 
 // fn application; tail flag triggers musttail under codegen.
 // `tail` is omitted when false (hash-stable when omitted).
+// `args` may be empty: a nullary call is the surface form
+// `(app f)` (resolution of Gitea #12). Read-tolerant: a JSON
+// document omitting the `args` key deserialises to `[]`.
 { "t": "app",  "fn": Term, "args": [Term...], "tail": false }
 
 { "t": "let",  "name": "<id>", "value": Term, "body": Term }
@@ -127,7 +130,10 @@ narrative — defaults, superclasses, diagnostics — lives in
 // `tail` triggers musttail (omitted when false).
 { "t": "do",   "op": "<eff>/<op>", "args": [Term...], "tail": false }
 
-// Ctor application; `args` omitted when empty.
+// Ctor application. `args` is always emitted on write (including
+// as `"args": []` for niladic ctors); reads tolerate the key being
+// absent and treat it as `[]`. This mirrors the read/write
+// asymmetry on `Term::App.args` (see above).
 { "t": "ctor", "type": "<id>", "ctor": "<id>", "args": [Term...] }
 
 { "t": "match", "scrutinee": Term, "arms": [Arm...] }