Update the 2021-era content, or caveat it properly

The migration (#37) deliberately did not touch the prose. It is now signposted with an "On dates" note on the home page, but that is signposting, not a fix.

The home page argues from a 2021 position in the present tense:

As of 2021, we are not following that pathway and hence net zero should already be earlier and reductions must start now and fast.

and works from the IPCC SR1.5 (2018) remaining carbon budget of 420 GtCO2 — which, five years of emissions later, is materially smaller.

The temperature chart is the exception: it now pulls HadCRUT5 through 2025, so the top of the page is current while the analysis below it is not. That gap is arguably worse than a uniformly dated page.

Options, roughly in order of effort:

1. Date-stamp the affected sections explicitly ("written 2021")
2. Refresh the carbon budget numbers against the latest IPCC/Global Carbon Budget figures
3. Rewrite the "What do we need to do?" section from where things actually stand now

Not migration work — this is writing, and needs a human who has the argument in their head.

Start with the home page, especially the “On dates” note, the 2021 present-tense argument, the 420 GtCO2 budget, and the “What do we need to do?” section. Review the cited IPCC SR1.5 and latest IPCC/Global Carbon Budget figures before choosing whether to date, refresh, or rewrite the affected content. Done means the prose and figures are either current or explicitly caveated, without leaving the updated temperature chart beside undated analysis.