Add blog post series (roadmap lean-rollout steps 1+2)

[noha]

Implements the "Serien (series)" section of Foliage Roadmap.md: posts can declare series: in frontmatter to belong to an ordered, finite sequence with its own overview page and inline navigation - distinct from a tag, which is unordered membership only.

New:

• FOBlogPost>>series/hasSeries reads the frontmatter key.
• FOSeries: slug + posts (a series' own reading-order post list), title derived from the slug (e.g. 'soil' -> 'Soil', 'byob-database' -> 'Byob Database') - a zero-config stand-in for a real title registry, deferred per the roadmap's lean-rollout ordering until actually needed.
• FOBlog>>series/seriesSlugs/postsInSeries:/seriesPostAfter:/ seriesPostBefore: - same grouping/sorting mechanics as the existing publishedPosts/postAfter:/postBefore:, filtered to a series and sorted ascending (reading order), not the blog's own newest-first browsing order.
• FOSeriesOverviewBuilder: same bodyHtml/abstract-concatenation approach as FOBlogOverviewBuilder, no pagination (series are expected to be short).
• FOBlogPostRenderContext: hasSeries/seriesTitle/seriesPartNumber/ seriesPartCount/seriesOverviewPath/seriesPreviousPost/seriesNextPost, for a template's inline "part N of M" nav.
• FOPublisher>>publish now also builds one page per series under blog/series/.html, alongside the existing overview pages and feed.

Deliberately not included (per the roadmap's own lean-rollout ordering, "step 3, if needed"): seriesPart frontmatter override for reading order, a slug->title registry, RSS per series.

Verified against real content: tagged the two real Soil posts with series: soil in a scratch copy and ran the full publish pipeline - blog/series/soil.html lists both abstracts in reading order (part 1 before part 2), and each post's rendered page (via the noha.github.io template's own new inline-nav section, done as a companion change in that repo) shows correct part-number/prev/next/overview-link output.