Drop the upper bound on user-defined function arity

[kaidesu]

Overview

Arity checking for user-defined functions (§14 decision 1) was stricter than the interpreter's own callback path. object.Function.Evaluate — what list.map/filter/reduce/each/sort call a callback through — never enforced a maximum argument count, so a callback like (item) => item * 2 already worked as map's argument even though map also passes an index and Ghost's own callers depend on this being fine. A direct call, though, was rejected for passing more arguments than a function declared parameters for, which meant a function could only rely on that permissiveness by accident of which call path invoked it, rather than as a rule of the language. This surfaced while updating Lumen, whose callbacks only survived because of that undocumented divergence between the two paths.

function greet(name) { return `hi ${name}` }
greet("Ada", "extra")   // before: argument error - `greet()` expects 1 argument, got 2
                        // after:  "hi Ada"

[1, 2, 3].map((item) => item * 2)   // already worked before (via list.map's own call path);
                                    // now the same holds for a direct call, too

The minimum-argument check is unchanged: a call that leaves a required parameter unbound is still an Argument fault.

Changes

Changed

• evaluator/function.go's checkArity no longer rejects a call for supplying more arguments than a function declares parameters for — only a missing required argument is an error now, checked with object.ArityAtLeast in every case.
• Updated SPEC.md (§8.7 and §14 decision 1) to describe and justify the revised behavior, and the §12 checklist entry that referenced "strict arity checking."

Removed

Fixed

Related issues

Additional context

Updated evaluator/evaluator_test.go: TestFunctionArity's exact/range-arity cases now expect the "at least" wording, and TestFunctionArityAllowsDefaultsAndVariety gained cases confirming extra arguments are accepted and dropped rather than rejected. go build ./... and go test ./... are clean.

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