Compute parent/child (session→daily→weekly→monthly) and prev/next
(chronological ordering within each level) edges at graph build time
from node metadata. Parse dates from keys for digest nodes (whose
timestamps reflect creation, not covered date) and prefer key-parsed
dates over timestamp-derived dates for sessions (timezone fix).
Result: ~9185 implicit edges, communities halved, gini improved.
Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>
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