Benchmark unsigned 64-bit multiplication

[connortsui20]

Adds a mul_u64_nonnull case to binary_ops. The suite covers Mul at every signed width and at u8, u16 and u32, but not at u64, which takes a different overflow check because it has no wider native type to widen into.

This lands separately so that CodSpeed records a baseline on develop before #9210 changes that check. Measured on that PR, the width gains 1.8x, which no existing benchmark would have caught.

[codspeed-hq]

Merging this PR will improve performance by 12.87%

⚡ 1 improved benchmark
✅ 1893 untouched benchmarks
🆕 1 new benchmark
⏩ 43 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	decompress[u64, (1000, 16)]	73.7 µs	65.3 µs	+12.87%
🆕	Simulation	mul_u64_nonnull	N/A	1.1 ms	N/A

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

---

_{Comparing ct/mul-u64-benchmark (7ae5392) with develop (4030e01)}

Footnotes

1. 43 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩