Switch README download chart to a cumulative-total trend

The old chart plotted weekly downloads independently, which made the
growth story noisy (one slow week reset the visual narrative). Switch
to a running cumulative total sampled at weekly boundaries so the line
is monotonically non-decreasing and the lifetime trajectory is obvious
at a glance. Default trend window extends to 26 weeks for a longer
horizon.

- Rewrite `.github/scripts/update_npm_download_chart.py` to bucket
  daily download rows into 7-day periods, then carry running totals
  (including pre-window history pulled in chunked range calls back to
  the package's first publish date) so the y-axis starts at the
  lifetime baseline rather than zero.
- Rename `.github/npm-weekly-downloads.svg` ->
  `.github/npm-total-downloads.svg` and update README + GitHub Actions
  references accordingly.
- Regenerate the SVG: current cumulative total ~10.3k downloads.

Co-authored-by: Open Codex <hff582580@gmail.com>

Author: LEON-gittech