Migrate Seq.js to TypeScript and re-parent the Seq hierarchy - #2210

Summary

Migrates Seq.js to TypeScript and re-parents the lazy-sequence hierarchy so each *SeqImpl extends its matching *CollectionImpl directly, replacing the runtime mixin(*SeqImpl, *CollectionPrototype) copies.

Stacked on #2209 (make-size-undefined), which it depends on: the typed *SeqImpl inherit size: number | undefined.

No runtime behavior change.

Migration

• Seq.ts (replaces Seq.js): KeyedSeqImpl extends KeyedCollectionImpl, IndexedSeqImpl extends IndexedCollectionImpl, SetSeqImpl extends SetCollectionImpl. SeqImpl stays as the kind-agnostic base (used by ConcatSeq). The IS_SEQ_SYMBOL brand, toSeq/to{Keyed,Indexed,Set}Seq, toString and cacheResult — previously inherited from SeqImpl — are now defined on each kind class so the re-parenting loses nothing.
• CollectionImpl.js: the three mixin(*SeqImpl, *CollectionPrototype) calls are removed — the kind-specific methods are now inherited through the prototype chain.
• Collection.ts: the lazy-cache dispatch moves into the base __iterate/__iterator (a present _cache is iterated, otherwise they delegate to __iterate*Uncached); to{Indexed,Keyed,Set}Seq become throwing placeholders the mixin overwrites, so the Seq subclasses can override them as (): this.

Because a *SeqImpl now is a *CollectionImpl, the Seq/collection-kind boundary casts disappear. The remaining casts are the Record d.ts boundary (dissolves when Record migrates) and noUncheckedIndexedAccess assertions on dense arrays.

Typing per the public d.ts contract

• partition narrowed per kind (3 *CollectionImpl + 4 *SeqImpl), concat declared on the kind Seq classes, conversion methods return the Seq types (toSeq(): Seq<K, V>, entrySeq(), keySeq(), …).
• Collection() / Collection.Indexed() / Collection.Set() callable without arguments, as the d.ts declares.
• Range() and RangeImpl.slice() return RangeImpl (a subtype of Seq.Indexed<number>). slice keeps the base : this contract: when it rebuilds through the concrete Range() factory it uses an as this cast — erased at compile time, the same cast the base hides inside reify/sliceFactory.
• makeSequence / collectionClass gain an unknown-kind base overload.

Type tests (ts-tests-src)

Un-skips the ts-tests-src source-pass duplicates now covered by the migration: seq.ts (3/4), empty.ts (2/2), range.ts (1/1 + a new #slice assertion), functional.ts (5/7), partition.ts (2/7) — tstyche actives go 212 → 225. The assertions reference the internal *Impl classes imported straight from the source until the public type names are emitted; each remaining skip documents its blocker (unmigrated List/Map/Record, mixin concat, d.ts-only types).

Docs

• .agents/commands/migrate-to-ts.md: adds a section on covariant this return types — the decision rule for leaf overrides (slice/reverse/sort/…) and when the localized as this cast is the right tool.
• CHANGELOG.md: notes the Range TypeScript migration under Unreleased (and moves the misplaced 5.1.9 entry below 6.0.0).

Test plan

• npm run type-check (3 projects) ✅
• npm run build ✅ (no module eval-order regression)
• npm run test:unit — 795 passed ✅
• npm run test:types (tstyche) — 888 assertions passed, 225 tests active / 197 skipped ✅
• eslint + prettier on changed files ✅