sourceFollow takes source.<branch> for every branch the seat lists (the answer with no branch), ApplyBranchSource computes each module's
fingerprint, inputs and graph node over a view in memory seeded from the branch's newest revision, and keeps it with KeepBranchEntries;
the trunk's modules, manifests, taken state, graph and packages are not written. The trunk branch is named once (index.TrunkBranch).
Builds: design 47 G §13, §14, §291, ADR 0666 §6, A-85, A-90, A-93
Claude-Session: https://claude.ai/code/session_01NcbgfGKaxw984XyAPfLm4h