[PRE-AUTHORED FIXTURE — represents the local rehydration step: the two
mock answers combined with the local repo context. No provider saw this document.]

hearthgrid-core fixes (rehydrated locally):

1. sync/delta_stream.py — apply provider A's fix option 2 (snapshot
   iteration with pop): flush_batch handles huge partner sync batches, so
   eager memory release matters for the Nortia Systems pilot volumes.
   Add A's suggested multi-entity unit test to tests/ as
   test_delta_stream_flush.py; note the single-entity blind spot in the
   test docstring.

2. Secret hygiene flag raised while preparing this task: settings.py
   commits a default HEARTHGRID_API_KEY. Rotate the example key and make
   the loader fail hard when the env var is missing. (This never left the
   machine — provider A saw only the snippet and the error.)

3. README API section — apply provider B's template to the public
   operations of billing/invoice_engine and sync/delta_stream. Keep
   partners/nortia_adapter.py and the conflict_resolver tombstone-grace
   strategy OUT of the public docs: the roadmap (ship before Veyra Labs,
   Q4 pilot expansion) and the differentiator description stay internal.

What each provider actually saw: a 10-line snippet with an error (A) and a
generic documentation task (B). Neither saw the repo name, the tree, the
committed key, the customer, or the roadmap.
