erlang: lists zip/2 + zipwith/3 + unzip/1 (841/841)

Zip family in lib/erlang/lists-ext.sx; length mismatch and malformed
pairs raise badarg. lists_ext suite 62 -> 70.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plans/erlang-on-sx.md

@@ -159,6 +159,8 @@ The Phase 9 opcodes are registered, tested, and bridged SX↔OCaml, but inert: n
 
 _Newest first._
 
+- **2026-06-30 stdlib hardening — `lists` zip family** — Added `zip/2`, `zipwith/3`, `unzip/1` to `lib/erlang/lists-ext.sx`. Length mismatch (zip/zipwith) and malformed/non-pair elements (unzip) raise `badarg` (port equivalent of Erlang's `function_clause`). `lists_ext` suite 62→**70** (+8, incl. a zip/unzip roundtrip). Conformance **833 → 841/841**. loops/erlang only.
+
 - **2026-06-30 stdlib hardening — `lists` flatten/max/min** — Added `flatten/1` (deep recursive flatten via `er-list-append`), `max/1`, `min/1` (full term order via `er-ext-lt?`, `badarg` on empty) to `lib/erlang/lists-ext.sx`. Gotcha caught: `er-ext-lt?` returns a raw SX boolean, so the extreme-finder uses it directly in `if` rather than wrapping in `er-truthy?` (which only recognises Erlang bool atoms, not SX booleans — the first cut wrapped it and silently never updated the running best). `lists_ext` suite 52→**62** (+10). Conformance **823 → 833/833**. loops/erlang only.
 
 - **2026-06-30 stdlib hardening — `lists` higher-order traversal** — Added `foldr/3`, `partition/2`, `takewhile/2`, `dropwhile/2`, `splitwith/2` to `lib/erlang/lists-ext.sx`, registered pure through the `er-register-builtin-bifs!` wrapper (consistent with the existing pure `map`/`filter`/`foldl`). `foldr` right-folds (order-preserving when consing); `partition` returns `{Satisfying, NotSatisfying}` order-preserved via `er-list-reverse-iter`; `splitwith` = `{takewhile, dropwhile}`. `lists_ext` suite 38→**52** (+14). Conformance **809 → 823/823**. loops/erlang only.