A structure benchmark, more wrangling samples, and story

A session aimed at getting the skills better by benchmarking them and adding samples, not just adding features.

structure benchmark. docs/structure-benchmark.md scores the wrangling skill across three real runs — the NOAA text file (comment lines, sentinels), the Project Drawdown markdown table, and a new one: eight legacy .xls workbooks of EIA oil prices. The oil-prices build is a ground-truth test — its output is content-identical to the long-running community datasets/oil-prices (the only difference is LF vs CRLF line endings). The benchmark turned up a real bug class the skill doesn't warn about — Excel serial dates are timezone-naive, and parsing them via JavaScript Date shifts every date by a day — plus four other prioritised skill edits, all filed as tracked issues. The edits aren't applied yet; the point of this pass was to gather the evidence first.

New datasets. oil-prices (Brent + WTI spot prices, four frequencies, from EIA). And co2-ppm gained the global CO₂ series, NOAA's published growth rates, and a decadal-mean table.

Project Drawdown moved to its own repo — https://github.com/datasets/project-drawdown.

Story craft, written down. docs/story-craft.md (what makes a good data story) and docs/voice-guide.md (the house voice — let the numbers carry it), drafted from the best practitioners and story #1.

Data story #2 begun. Planetary Boundaries — source snapshot archived and the outline + chart plan committed, the #10 way (argument signed off before prose). Prose still to come.