A header included by fourteen translation units was hashed once per dependent,
and every built action hashed its inputs twice: once to decide it was stale and
again to store the new key. Digests are now memoised for the run, and the
scheduler drops the entries for an action's outputs and depfile once it
finishes, so a freshly written file is never read from the memo.
Digests every input plus the exact argument vector into one key, persisted
under the build directory. Staleness becomes a content question rather than a
timestamp comparison.
milner