Forward pipeline context, add other_context to JoinTransform, drop SerializeAsAny

- NarwhalsPipelineModel.__call__ now forwards the caller's context to
  source(context) rather than passing NullContext, so context-keyed sources
  (e.g. TPCHDataGenerator with TPCHTableContext) can be used directly without
  per-table adapter wrappers.

- JoinTransform gains an other_context field so a single context-keyed source
  can be reused across multiple joins (e.g. one TPCHDataGenerator instance
  serving customer/orders/lineitem). Validator now checks
  isinstance(other_context, other.context_type) at construction.

- Drop SerializeAsAny from source and other fields. ccflow's BaseModel
  metaclass already wraps bare BaseModel-typed fields, so the explicit
  annotation was redundant. Verified subclass info still survives JSON
  round-trip in tests.

- Loosen SequenceTransform.transforms to accept the same
  Union[NarwhalsFrameTransform, Callable] as NarwhalsPipelineModel.transforms,
  for consistency. Beef up the comment on NarwhalsFrameTransformOrCallable to
  spell out why both branches matter (BaseModel branch enables type_-based
  round-trip; Callable branch is a runtime escape hatch that doesn't
  serialize).

- Notebook polish: drop hardcoded TOC, drop graph-awareness subsection, drop
  trailing Pointers section, fix ascii alignment in section 7 diagram, and
  remove the TPCHTableProvider adapter (and per-table providers) by leaning
  on context passthrough -- pipelines now use the generator directly. Combine
  the AggregateByReturnStatus and SortByReturnStatus transforms into a single
  SummarizeByReturnStatus (group keys = sort keys, naturally one operation).
  Add a new section 4 'Aside' that frames NarwhalsFrameTransform as an
  opt-in convention rather than a requirement, with a tradeoffs table for
  plain function vs plain BaseModel vs NarwhalsFrameTransform.

- Tests: add coverage for plain callable + plain ccflow.BaseModel inside
  SequenceTransform (round-trips via type_), context forwarding to source,
  other_context flow-through, and other_context type-mismatch rejection
  (38 tests total, full suite 687 passed).

Signed-off-by: Pascal Tomecek <pascal.tomecek@cubistsystematic.com>

Author: ptomecek