Add callback/event support for operator splitting integrator (Fixes #36)

[ChrisRackauckas-Claude]

Summary

This PR implements callback and event support for OperatorSplittingIntegrator, addressing issue #36.

• Add full support for discrete callbacks at the outer integrator level
• Add simplified support for continuous callbacks (endpoint-only checking)
• Implement proper u_modified! tracking for solution modifications
• Add savevalues! function for callback-triggered saves
• Add comprehensive test suite for callback functionality

Design Decisions

As discussed in issue #36, there are two main challenges for callbacks in operator splitting:

1. Which integrator triggers which callback: We handle callbacks ONLY at the outer integrator level, AFTER all subproblems are solved for a timestep. This ensures callbacks have access to the full solution state.

2. Visibility of unknowns: By processing callbacks at the outer integrator level, callbacks can see the full solution u, not just partial views that individual subintegrators work with.

For continuous callbacks, full root-finding across split subproblems is not well-defined, so we use simplified endpoint checking (detect sign changes between tprev and t). Users requiring precise event timing should use smaller timesteps or ensure their callback conditions align with the timestep boundaries.

Test plan

• Discrete callback with simple condition - triggers correctly
• Discrete callback that modifies u - modification propagates correctly
• Discrete callback with save_positions - saves work correctly
• Multiple discrete callbacks - all callbacks execute
• Continuous callback zero-crossing detection - basic detection works
• Continuous callback with reflection - affect function executes
• Callbacks with nested splitting - works with recursive structures
• u_modified! functionality - flag is properly set/cleared
• Callback preserves solution accuracy - no-op callback doesn't change solution

All tests pass locally.

cc @ChrisRackauckas

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