Guard rasterization against int32 intersection overflow

[jiagaoxiang]

Summary

• Add host-side validation in rasterization entrypoints to reject intersection buffers that exceed int32 indexing limits before kernel launch.
• Return a clear runtime error instead of allowing int32 overflow to trigger GPU memory access faults.
• Document the failure signature, root cause, mitigation, and 64-bit functional-fix path in a dedicated how-to doc.

Test plan

• Reproduce the crash with synth_rast_bwd_test.py on ROCm.
• Validate patched gsplat raises RuntimeError guard instead of GPU memory fault.
• Confirm PR diff vs release/1.5.3b2 contains only the guard and docs updates.

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