fix: identity equality for expression nodes; eager equijoin collision check

[Xof]

P2 API/semantics fixes (closes A1, A2)

A1 — expression nodes had value-equality enabled, and it raised

The node dataclasses used the default eq=True. The generated structural
__eq__ compares fields, reaching Attr.__eq__, which returns a Predicate
whose __bool__ raises PredicateError. So any of these blew up:

node1 == node2          # PredicateError
node1 in [node2]        # PredicateError
set([node1, node2])     # PredicateError on collision

eq=True + frozen=True also synthesized a __hash__ over the fields,
making Grouping and Rename (dict-valued fields) unhashable —
hash(node) raised TypeError.

Expression-tree equivalence is identity, not structure (the engine check
uses is; Attr.__hash__ already chose identity). Fix: eq=False on all
12 node dataclasses, so __eq__/__hash__ fall back to object identity.

A2 — Equijoin deferred its collision check past construction

Equijoin.__post_init__ validated only that the join attributes exist; the
ambiguous-output-name SchemaError lived in _schema() and surfaced only on
a later .schema() call — unlike every other node, which validates eagerly.

Fix: move the collision check into __post_init__ (byte-identical logic),
leaving _schema() to only shape the result schema.

Tests

• TestNodeIdentity (7): == is identity (self True, distinct False, no
  raise) even with predicate-bearing children; membership checks don't raise;
  Grouping/Rename are hashable and identity-keyed; a guard asserts every
  one of the 12 node classes inherits object.__eq__/object.__hash__.
• TestEquijoinEagerValidation (2): colliding non-join column raises
  SchemaError at construction; the dropped right join column is not a
  false collision.

Local gate green: ruff, mypy, pytest (106 passed, 15 skipped).