cook build web: search-index.js changes on every build of an unchanged collection #806

cook build web doesn't write the same site twice for the same collection: static/search-index.js comes out with its entries in a different order on every run. So a static site published through git, rsync or a CI deploy always has a change to ship, even when nothing changed.

To reproduce: build the seed twice into two folders and compare them.

cook build web --base-path seed /tmp/a
cook build web --base-path seed /tmp/b
diff -rq /tmp/a /tmp/b

Only static/search-index.js differs: same entries, different order. Everything else is stable, including --sitemap and --feed output.

The index is built by walking the recipe tree (src/build/index.rs), whose children are a HashMap, so the order depends on the hash seed of each run. The sitemap (src/build/sitemap.rs) already sorts its entries by path for this reason. This has been there since the static-site generator came in, in f6703df (#344).