Function compute_file_scaffold_hash 

pub fn compute_file_scaffold_hash(token_stream: &[u8]) -> ContentHash

Hash the file’s scaffold: the residual non-definition top-level token stream (every leaf under the file root not covered by an extracted symbol’s span). This is what binds use-decl swaps, impl Trait headers, attribute edits, macro_rules! bodies and definition-free files (re-export-only libs, top-level statements) into the file digest — semantic content that lives outside any extracted symbol.

Canonical layout hd-sem-scaffold-v1: the length-prefixed leaf token stream (same framing as a symbol hash’s token stream), comments excluded.