Update investigation notes with full neuron_sample comparison and benchmarks
Rewrite static_sample_investigation.md with:
- Context: goal is to determine neuron-only vs general static path
- Methodology: align on newest _sample algorithm, not neuron_sample fork
- Full comparison table: _static_sample vs neuron_sample (14 items)
- Benchmark results for Items A (output_ids CPU) and B (4D mask)
- Recent PRs affecting _sample (#44226, #44130, #44181, #44126)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>