Add the public InfiniRT event bridge

[voltjia]

Problem

torch-infini has no public event abstraction for dependency tracking, timing,
or stream coordination.

Scope

• Wrap InfiniRT event create, record, query, synchronize, elapsed-time, and
  destroy operations.
• Provide a PyTorch-compatible torch.infini.Event surface.
• Implement stream wait-event behavior with correct lifetime ownership.
• Add CPU capability/error tests and NVIDIA behavioral tests.
• Do not change allocator or operator behavior in this change.

Acceptance criteria

• Events can be recorded on nondefault streams and queried without blocking.
• synchronize, elapsed_time, and wait_event follow PyTorch semantics.
• Unsupported backend capabilities fail clearly.

Depends on #6.