interp: guide the [x y z] vector trap and add a signature builtin

[ecto]

What

Two ergonomic fixes in loon-lang, surfaced by loon's first end-user-facing consumer (the vcad Typst plugin, where people write loon inline):

1. The [x y z] vector trap gets a guiding message

[30 20 -1] is application, not a vector literal, so it evaluated to the opaque not callable: 30. All four not-callable sites — the interp call path, the pipe path, machine.rs, and builtins::apply_value — now share a single not_callable_msg helper. For a numeric callee it names the cause and shows both correct spellings:

not callable: 30 — 30 is a number, not a function. loon has no [x y z] vector syntax; write #[30 ...] for a vector, or pass components as separate arguments, e.g. [translate 30 20 -1 solid]

Non-numeric callees keep the plain not callable: <v> message. The error already carries a span via err_at; a companion vcad-loon change (separate repo) threads that span to a user-source line number.

2. signature builtin for discoverability

No way previously to learn a symbol's arity/arg names without reading source. [signature translate] → "[translate x y z s]". Multi-clause fns join with |, rest params render as & name, builtins report "[name ...] (builtin)". Registered in the type checker as ∀a. a → Str (keeps the interp/checker builtin-parity test green).

Why

These are the language-level halves of a three-problem ergonomics pass (full tradeoff writeup, including the vcad-side subject-last lint and span→line threading, is in docs/plans/2026-07-18-subject-last-ergonomics.md). The design bar for the vector trap: make the wrong-input mistake loud and self-correcting at the moment it happens, without a parse-time ban that would outlaw legal macro-generated forms.

Tests

cargo test --workspace is green. New in tests/interp_tests.rs:

• numeric-callee message on both the tree-walker and the EIR VM
• plain message preserved for a non-numeric callee
• signature over named, anonymous, builtin, and non-fn values

Reviewer notes

• Backward compatible: no syntax or API changes, only a richer error string and one new builtin.
• The doc references vcad extensively for context, but this PR touches only loon-lang. The vcad-loon consumer changes (VcadError line numbers, lint_vcad for the subject-last B−A trap) land in the vcad repo separately.

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Project	Deployment	Actions	Updated (UTC)
loon	Ready	Preview, Comment	Jul 18, 2026 8:50pm