tools: add one-command datapoint runner

[gouravkargwal]

Adds c/tools/datapoint.py — a stdlib-only, one-command runner for the standard machine datapoint that docs/benchmarks.md asks for (engine load, capped greedy decode tok/s, iobench disk figures), printing a ready-to-paste markdown block.

What it measures, and how:

• Exact decode tok/s — generation is capped at --max-new tokens, so the token count is known without re-tokenizing output. TEMP=0 greedy.
• True cold vs warm — before measuring, the page cache is evicted (macOS purge if permitted, otherwise a portable temp-file write larger than RAM). iobench cold runs right after eviction, before decode re-warms the container. --no-evict skips it.
• Disk — iobench 19 MB × 64, 8 threads, cold + buffered, with the macOS F_NOCACHE caveat [Bug]: iobench test command output misleading #86 called out in the emitted block.
• Machine info auto-detected (CPU, RAM, cores, OS) per platform.

docs/benchmarks.md Quick Start now points at it as step 3 (steps renumbered).

The M3 datapoint row it produced is in a separate PR (docs: add Apple M3 16 GB OLMoE datapoint row, #951).