Summary:

• preserve keyed/indexed/set sequence prototypes when concat creates a multi-collection concat sequence
• keep the existing flattened concat iteration behavior shared across concat sequence variants
• add a regression test that Seq.Indexed(...).concat(...).zip(...) remains available

Tests:

• npm_config_cache=/tmp/immutable-npm-cache npx jest __tests__/concat.ts __tests__/zip.ts --runInBand
• npm_config_cache=/tmp/immutable-npm-cache npm run test:unit -- --runInBand
• npm_config_cache=/tmp/immutable-npm-cache npm run lint
• npm_config_cache=/tmp/immutable-npm-cache npm run type-check
• npm_config_cache=/tmp/immutable-npm-cache npm run build
• git diff --check

Fixes #2161