Each finding from .agents/migration-status.md was traced back to its
originating PR, then fixed or confirmed as an intentional change:

- R1 (isSubset string argument, from immutable-js#2204): keep the new
  character-iteration semantics, consistent with isSuperset and with
  Collection('abc'); documented as a v6 breaking change in the CHANGELOG
  and pinned by multi-char unit tests.
- R2 ('@@iterator' fallback removal, from immutable-js#2127): confirmed intended
  modernisation; explicit CHANGELOG entry, and the two immutable.d.ts
  docstrings no longer mention @@iterator. immutable.js.flow is left
  untouched since Flow's @@iterator syntax denotes Symbol.iterator.
- R3 (reverseFactory reverse-iteration keys, ?? 0 from immutable-js#2206, NaN keys
  already in v5): fixed by counting down from the number returned by
  ensureSize(collection) - the reversed sequence has the same size as
  its source. Regression tests cover both __iterate (reduceRight) and
  __iterator (keyed reverse) paths; CHANGELOG entry since the NaN
  behaviour shipped in v5.
- R4 (wholeSlice with begin === undefined, from immutable-js#2128): restored the
  faithful v5 form (begin !== undefined && begin <= -size) so slice()
  with no arguments goes through sliceFactory again, on empty
  collections too; identity regression test added.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0122shfP2ngkkjuXZBWoeFGW