docs: refresh instrument grid comments

[bout3fiddy]

Summary

• refresh instrument-grid boxed comments for file headers, memory layouts, and instrumentation blocks
• correct layout sizes and field offsets from compiler-backed @sizeOf, @alignOf, and @offsetOf probes, including trace-phase timing variants
• add styleguide guidance for layout comments and keep local scratch docs out of commits

Verification

• uv run scripts/linting/check-zig-styleguide.py on all changed Zig files: no findings
• git diff --check: passed
• zig build check: passed
• prek run --all-files: passed; advisory hooks still print existing repo-wide findings outside this branch

Layout Math Checked

• InstrumentGridSummary: 80 B
• InstrumentGridProduct: 240 B
• InstrumentGridProductView: 248 B
• Buffers: 216 B
• ProductStorage: 552 B normal, 560 B with trace phase timing
• RunningSummary: 48 B
• KernelStorageBuilder: 80 B
• ForwardSampleScratch: 3304 B normal, 3320 B with trace phase timing
• ForwardPrefetchWorker: 184 B normal, 192 B with trace phase timing