Brummel
509c9700bf
Records the runtime refcount-1 dispatch design (Lean 4 / Roc lineage), why the dispatch lives at codegen rather than in runtime/rc.c (per-site decision, no new ABI), the new reuse_shape pre-codegen pass with its five reason sub-codes, and — crucially — the explicit field-cleanup scope cut. The reuse arm does not dec old pointer-typed fields because the canonical fixture's pattern-bound subterms alias the new field values (`(reuse-as xs (Cons _ (map_inc t)))` returns t's in-place-rewritten box as the new tail). The root cause is upstream: pattern-matching does not null out source slots. 18d.2 ships the perf-only half (alloc + outer cascade elided); 18d.3 / 18e closes the leak with move-aware patterns.