Integrate blog index generation into dune promotion workflow
gen_blog_index.exe now writes into _blog_gen/ directory (instead of
overwriting in place) and runs from site/ cwd. dune.inc gains a
(dir _blog_gen) rule plus per-index diff rules under @runtest, so
`dune runtest site/` detects stale indexes and `dune promote` updates them.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>