[NelsonVides]

Follow-up to #377. When commit_zone/1 or discard_zone/1 gets a zone whose staging entry is gone and whose generation no header names any more, it dropped that generation at once, taking it for one that was dropped before it was ever published. It can also be one that was published and then replaced, by a later commit or by delete_zone/1, whose drop is already scheduled for the end of the grace period because queries that read its header may still be resolving against it, for example while a pipeline stage waits on an upstream lookup. A late discard or commit deleted its records under those queries, which then got NXDOMAIN, the answer the grace period is there to prevent.

• Only a caller that takes a generation's staging entry drops it at once. Publishing takes that entry first, so taking it proves nothing has published the generation and no lookup can be reading it.
• Any other drop waits out the grace period, as a replaced generation's does. That still drops whatever was written into a generation that was dropped before it was published, 30 seconds later than before.

Return values are unchanged: committing a zone that was replaced since it was published already returned {error, not_staged}, and its documentation now says so. Discarding or committing a zone that is still live is still a no-op.

🔍 QA

Scenario: a late settle of a replaced zone leaves it readable

• Run rebar3 ct --suite=test/zones_SUITE --group=cache.
• Expect replaced_zone_outlives_a_late_settle to pass: after one zone replaces another and a second is deleted, discarding and committing each of them again leaves both resolving through their headers. Against v12.0.0 the replaced one answers NXDOMAIN.

Scenario: a dropped staged zone still loses what was written into it

• Expect staged_zone_is_dropped_with_its_owner to pass: records written into a zone dropped with its owner are gone once the grace period has passed.

📋 Deployment Pre/Post tasks

• POST: release v12.0.1 to hex.
• POST: bump erldnsimple to v12.0.1.

Deployment Verification

• make test passes.