Parse zero-argument contract signatures () -> T

[yavorpanayotov]

What

Lets contract operation signatures declare zero arguments with an empty parameter list:

contract Registry {
    list_things: () -> Set<Foo>
    health: () -> Status
}

Closes juxt/allium#37.

Why

Signatures previously required ≥1 parameter — both op: () -> T and op: -> T failed to parse:

expected expression (identifier, number, string, ...), found ')'

That forced a nominal dummy parameter (op: (_unit: Any) -> T) for common parameterless operations (list-all, fetch global state, health checks), leaking an implementation artefact into the spec and making signature-level comparison against a real API noisier.

Fix

parse_paren_expr only recognised (name: ...) typed parameter lists; empty () fell through to the expression parser, which errored on ). It now handles () as an empty parameter list — represented as an empty block — so the trailing -> Type projection attaches the return type with no parameters, exactly mirroring how multi-parameter lists already become a block.

The empty-params form needs no downstream changes: I traced the consumers and both the analyzer (analysis.rs) and the test-plan builder (test_plan.rs) treat contract signatures as generic expression trees and never extract/validate parameters, so an empty parameter list flows through cleanly.

Scope

• () -> ReturnType is supported (the conventional spelling).
• The bare-arrow form -> ReturnType remains a parse error — keeping one canonical spelling, as the issue allowed ("at least one of the two forms").

Verification

• New regression test zero_arg_contract_signature_parens asserts the form parses cleanly and produces an empty-block parameter list.
• cargo test --workspace green (579 tests; parser 392, CLI 187).
• End-to-end: allium check on the issue's exact reproduction now exits 0 with no diagnostics (previously a parse error).
• The LSP/editor path parses via the same Rust parser compiled to WASM, so it picks this up once the WASM parser is rebuilt — no separate TypeScript change needed.

Docs: added a zero-argument signature example to the v3 language reference (contracts section).

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