Migrate mixin operations to real TypeScript class methods in Collection.ts - #2193

Summary

Follows #2192 and builds on the Operations.js split (#2194). Migrates a large
batch of mixin methods out of CollectionImpl.js into real TypeScript class
methods on the Collection.ts classes — a concrete step toward deleting
src/utils/mixin.ts.

What changed

• Real class methods (no longer mixin entries) on CollectionImpl /
  KeyedCollectionImpl / IndexedCollectionImpl: map, toSeq, filter,
  filterNot, partition, reverse, slice, sort, sortBy, flatMap,
  flatten, max/maxBy/min/minBy, skip/skipLast/skipWhile/skipUntil,
  take/takeLast/takeWhile/takeUntil, butLast, rest, count,
  findLast/findLastEntry/findLastKey/findLastIndex, lastKeyOf/lastIndexOf,
  last, keySeq/valueSeq, flip, findIndex/indexOf, interpose/interleave,
  zip/zipAll/zipWith. Implementations delegate to the operations/factories

  • operations/helpers functions (import-safe — no eval-time extends, no Map
    import, so no load-order cycle).

• Removed the temporary interface declaration merging for map/toSeq: the
  real method bodies now carry the types directly.

• IS_*_SYMBOL brands are real runtime props assigned on the prototypes in
  Collection.ts (IS_COLLECTION_SYMBOL, IS_KEYED_SYMBOL, IS_INDEXED_SYMBOL,
  IS_ORDERED_SYMBOL). They make the otherwise structurally-identical empty
  subclasses distinct, which fixes this collapsing to never in toSeq's
  isIndexed/isKeyed narrowing, and they are what isKeyed/isIndexed/… test
  at runtime. They must live on the prototype (not as class fields) because
  makeSequence uses Object.create(prototype) and bypasses the constructor.

• Every overload is preserved (e.g. filter/partition type-guard overloads,
  flatMap indexed-vs-keyed, flatten number-vs-boolean, the zip family arity
  overloads), with a single loose implementation signature — matching the
  hand-written type-definitions/immutable.d.ts.

• Migration guide updated (.agents/commands/migrate-to-ts.md): added a step
  documenting overload preservation.

CollectionImpl.js shrinks accordingly; the 3 mixin(...SeqImpl, ...) calls and
the conversion/aggregation methods that still depend on operations/sequences.js
(eval-time extends) and operations/aggregations.js (imports Map) are
intentionally left for a later PR.

Tests

• Unit — behavioral tests live in the instance files where the shared methods
  are exercised through concrete types: __tests__/List.ts (indexed helpers +
  edge cases), __tests__/Map.ts (keyed variants), __tests__/toSeq.ts.
• Type (TSTyche) — type-definitions/ts-tests/collection.ts covers return
  types and overloads across List/Map/Set/Seq, including type-guard narrowing,
  the zip tuple arities, flip, and error cases (indexed-only methods on keyed
  collections).
• src/Collection.ts line coverage ~97% (remainder = abstract __iterate/
  __iterator stubs, unreachable from the public API).

Test plan

• npm run type-check — 0 errors
• npm run test:unit — green
• npm run test:types — green
• npm run lint / npm run format — clean