Arrowed lanes must follow the step number, not the file

The docs-site flow extractor turns a skill's step headings into a lane rendered with arrows. An arrow asserts sequence, but the extractor emitted headings in file position. cover/SKILL.md declares Step -0.5 after text about Step -1's place in the run order, so the arrows claimed an order the numbering contradicts. Toggle the two behaviours below.

Pre-flight lane, cover/SKILL.md headings

Only the two heading-derived lanes (pre-flight and core-fallback) are sorted by parseFloat of the step number. A table-derived core lane keeps its row order: there the author's row sequence is the declaration, and its first column is not always numeric. The regression test pins both behaviours (docs/site/__tests__/skill-flow.test.tsx).