Add unused forward_interp_safe reference implementation

Double-where safe-division alternative to the JITTER approach — not
yet wired in, see admin_jammy/prompt/autoarray/rectangular_interp_custom_vjp.md
for the follow-up task that evaluates and switches to it.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: 2 people