Bundle expectations: per-source deltas that compose across parallel PRs

Filed from #137's review: four UK-wave PRs (#137, #139, #141, #155) branch from the same base and each rewrites the absolute totals in tests/test_chronicle_bundle.py (fact_count, source_package_count, per-entity and per-source splits). Each is right on its own branch and stale the moment a sibling merges — so the current shape costs one hand-recomputed rebase per sibling merge, and the tempting conflict resolution (take one side's numbers) yields a green test that has stopped checking anything.

Proposal: keep the whole-bundle snapshot as a final tripwire, but move the load-bearing assertions to per-source (or per-package) counts:

• a by_source mapping asserted key-by-key, so a PR touching only slc and a PR touching only obr compose without conflict — their diffs to the mapping are disjoint;
• derived totals (sum of the mapping) asserted against the snapshot, so drift between the two representations still fails loudly;
• per-package fact counts inside each source's packages, so an accidental record-set drop inside one package can't hide in a source-level aggregate.

A sibling merge then rebases with zero constant edits when sources are disjoint (the common case), and a genuine collision surfaces as a real semantic conflict on one mapping line instead of four unrelated integers.

Current cost is real: this wave alone has done four hand re-measurements (~13-minute suite runs each) that per-source deltas would have made no-ops.

🤖 Generated with Claude Code

Start in tests/test_chronicle_bundle.py and inspect the existing whole-bundle, per-entity, and per-source assertions. Run that test file before and after reorganizing expectations around per-source and per-package counts. Done means disjoint source changes avoid constant conflicts while derived totals and the whole-bundle snapshot still catch drift or dropped records.