Add a series index page listing all series with abstracts

[noha]

Per feedback on the initial series feature: series entries should render through a real template (like blog-post-abstract does for posts) instead of hardcoded Smalltalk HTML, and multiple series in a listing should sort by recency rather than alphabetically, matching how the blog overview itself already orders posts.

• FOSeries gained website/abstractLayout/abstractTemplate/renderAbstract, mirroring FOBlogPost's own abstract-rendering shape - renders through a new 'series-abstract' template (companion change in noha.github.io) via the new FOSeriesRenderContext (title/pathString/abstract/partCount).
• FOSeries>>abstract reuses the first (reading-order) post's own already- truncated abstract text - same zero-config reasoning as titleFromSlug:.
• FOBlog>>series now sorts by each series' most recent post's publishDate descending (posts are stored reading-order-ascending, so that's simply the last one), not alphabetically by slug - alphabetical was only ever meant for deterministic per-series-file generation, not as a real reading order for a listing of multiple series.
• FOSeriesIndexBuilder builds one page concatenating every series' rendered abstract, same bodyHtml/outer-template approach as FOBlogOverviewBuilder/FOSeriesOverviewBuilder.
• FOPublisher>>publish inserts it at blog/series/index.html, alongside the existing per-series overview pages.

Verified against the real noha.github.io content: blog/series/index.html lists the "soil" series with the correct link, first-post abstract, and part count. Multi-series ordering (recency, not alphabetical) is only exercised by synthetic tests so far, since real content currently has just one series.