ADR-0184 §7 S4: add generate/derivation/state/source.py — the single
boundary through which semantic-ledger worlds become pool candidates —
and swap pool.py's accumulation source to semantic_state_candidates().
accumulate.py keeps its public surfaces as thin compatibility wrappers
(ADR-0184 §10). Byte-identical over the 937-problem differential;
verify.py / pool.py remain the sole commit authority (structural
no-authority-import scan over state/, non-vacuous).
Also: docs/analysis design note for the S3–S5 boundary stack, and a
drive-by ruff F541 fix in r1_reconstruction.py (rf->r, zero behavior).
