Add lazy superoperator types and measurement utility functions

[Krastanov-agent]

Summary

• Add AbstractLazySuperOperator hierarchy with LazyPrePost, LazySuperSum, LazySuperTensor types for efficient lazy evaluation of superoperators
• Add measurement utility functions: _drop_singular_bases, _branch_prob, _overlap, _project_and_drop for quantum measurement post-selection
• Uncomment expect function for AbstractKet types that was marked TODO upstream

New Types

• AbstractLazySuperOperator: Base type for lazy superoperator implementations
• LazyPrePost: Applies pre- and post-operators without storing full superoperator matrix
• LazySuperSum: Lazy sum of multiple superoperators with factors
• LazySuperTensor: Lazy tensor product of superoperators for composite systems

Utility Functions

• _drop_singular_bases: Remove dimension-1 bases after measurement
• _branch_prob: Calculate measurement outcome probabilities
• _overlap: Compute state/operator overlaps
• _project_and_drop: Project onto measurement outcome and reduce dimensionality

Test plan

• Verify all new types can be constructed and used
• Test lazy superoperator arithmetic operations work correctly
• Confirm measurement utilities produce expected results on example systems
• Check that exports work and no name conflicts exist

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