erlang: fold lists/proplists BIFs into transpile.sx + runtime.sx (completes 27dedf9b)

27dedf9b deleted lists-ext.sx but a botched git-add left the actual
fold-in unstaged, so HEAD briefly referenced functions that weren't
registered. This commit lands them:

- transpile.sx: function bodies appended next to existing er-bif-lists-*
- runtime.sx: registrations inside er-register-builtin-bifs!
- conformance.sh: drop the lists-ext.sx load; keep the lists_ext suite

Conformance 874/874 (verified on the combined tree before push).

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 retire separate `lists-ext.sx` — fold stdlib BIFs into canonical files** — The 8 stdlib commits had lived in a standalone `lib/erlang/lists-ext.sx` (a workaround for this worktree's broken sx-tree write tools). Folded the function bodies into `transpile.sx` (appended, alongside the existing `er-bif-lists-*`) and moved the registrations **directly inside `er-register-builtin-bifs!`** in `runtime.sx` — so they now reach every erlang consumer (fed-sx, identity, …) that loads the runtime, not just the conformance harness. The `er-register-builtin-bifs!` **wrapper trick is gone** (no longer needed once registrations live inside the function the registry-reset re-runs). Deleted `lists-ext.sx` and its `(load …)` from `conformance.sh`; kept the `lists_ext` test suite + wiring. Byte-exact splice (no transcription), `sx_validate` clean, conformance **874/874** unchanged. Mirrors the same fold-in landed on the `architecture` branch (commit `39dbb00c`, equivalence verified: identical test suite + function bodies). loops/erlang only.
+
 - **2026-06-30 stdlib hardening — `proplists` module** — Added `proplists:get_value/2,3`, `get_all_values/2`, `is_defined/2`, `lookup/2`, `delete/2` to `lib/erlang/lists-ext.sx` (header widened to "lists + proplists"). Property-list semantics: a bare atom `A` is shorthand for `{A, true}`, a tuple's first element is the key, lookups use the first match. `lookup` returns the tuple (or `{Key,true}`) or `none`; `get_value` defaults to `undefined`. The `lists_ext` suite (counter trio `er-lx-*`, now spanning both modules) 91→**103** (+12). Conformance **862 → 874/874**. loops/erlang only.
 
 - **2026-06-30 stdlib hardening — `lists` flatmap/filtermap/mapfoldl/search** — Added `flatmap/2`, `filtermap/2` (`true` keep / `false` drop / `{true, V}` transform), `mapfoldl/3` (returns `{MappedList, AccFinal}`), `search/2` (`{value, E}` | `false`) to `lib/erlang/lists-ext.sx`. `lists_ext` suite 83→**91** (+8). Conformance **854 → 862/862**. The `lists` module is now broadly covered (sort/usort/keylists/fold/partition/while/flatten/min/max/zip/slicing/flatmap/filtermap/mapfoldl/search on top of the originals). loops/erlang only.