5d: document the seed↔overlay boundary and add a drift check. core fns split
across a Janet seed (core-X, registered in core-bindings) and a Clojure overlay;
five names (char?/sorted?/sorted-map?/sorted-set?/transduce) carry a defn in
both, with the overlay copy authoritative and the seed copy internal-only. The
into-vs-transduce home asymmetry was undocumented. Adds docs/seed-overlay-registry.md,
SEED-TWIN: comments at the five seed sites, and a build-time drift check
(test/unit/seed-overlay-registry-test.janet) that recomputes the twin set from
source and fails if it diverges or a twin leaks into core-bindings.
5e: rep↔API pointer comments in pv/plist/phm/phs/lazyseq (representation lives
here; Clojure-facing ops dispatch in core_coll/core_types) and back-pointers in
core_coll. No behavior change — comments, docs, one source-analysis test.
Full gate green (suite ≥4695 pass / ≥88 clean files), drift check passes.
