Function derive_section_key 

pub fn derive_section_key(heading: &str) -> String

Derive the storage key a ## Heading line resolves to.

The single owner of the heading→key mapping: the entity parser uses it to place parsed section content, and the schema loader’s round-trip check uses it to refuse schemas whose declared headings could never find their way back to their declared keys. A second copy of this logic is how silent section forks return — both sides must call this function.

The mapping is deliberately narrow: lowercase the heading, replace spaces with underscores. Anything looser (slugging punctuation, folding diacritics) would let two distinct declared sections collide on one key, trading a visible refusal for an invisible content merge.