Support Cartesian products with more than two inputs

[quinnj]

product(f, a, b, c, ...) currently throws a MethodError. The added method combines three or more inputs using Julia's existing Cartesian-product iterator, preserving the combined axes, inferred element type, and the first input's array allocation behavior. The docstring and README show the additional-input form.

Fixes #58.

The workflow also includes the same scoped, conditional OIDC authentication and strict upload-error handling as #60, #61, and #62. The existing Codecov uploader rejects unauthenticated uploads to protected branches; these settings make the coverage upload verifiable.

Validation:

• Both unchanged baselines passed the two-input control and rejected all 13 additional-input checks. The candidate passes all 14 public checks on Julia 1.10.12 and 1.13.1.
• Both complete four-thread, bounds-checked native suites pass 227 assertions, including 13 new permanent checks for argument order, three to five inputs, mixed dimensions, empty inputs, missing values, inference, and BitArray output.
• Five additional OffsetArrays 1.17.0 interoperability checks pass on both runtimes, covering shifted axes, zero-dimensional inputs, empty inputs, and output values. All frozen tracked-file hashes match the reviewed candidate.

Exact-head CI: all four native jobs pass, including nightly and 32-bit Windows. Every raw log confirms its native suite passed and Codecov accepted the correct JuliaData repository and full commit SHA. All four reports are processed (73.64% overall; patch 100%); all six checks are green. The PR still requires an independent GitHub approval.

Co-authored by Codex

AI disclosure: This work was prepared with assistance from OpenAI Codex.