fix(query): never answer from a secondary index that is still building

[whilo]

A schema-declared secondary index is created with status :building and backfilled by the writer. A database built with d/db-with has no connection and therefore no writer, so the index stays :building indefinitely and accumulates only the datoms of transactions made after it was declared — while the aggregate path never looked at the status.

(def db (d/db-with (db/empty-db {:num/v {}
                                 :idx/a {:db.secondary/type :stratum
                                         :db.secondary/attrs [:num/v]}})
                   [{:db/id 1 :num/v 10} {:db/id 2 :num/v 30}]))

(d/q '[:find (min ?x) :where [?e :num/v ?x]] db)   ;; => [[nil]]   reference: [[10]]
;; …after one more transaction adding 50:
                                                   ;; => [[50]]    reference: [[10]]

A silent wrong answer on any column type, not a stale one, and independent of the open stratum questions.

The fix

Skip an index whose :db.secondary/status is :building or :disabled — the same check the transaction path already makes. The query then falls back to a path that reads the primary indices and is correct.

Why this gate and not another

:building separates the broken construction route from every working one exactly. Verified on all three:

route	status	delegate	result
hand-assembled assoc :secondary-indices (what every existing test uses)	nil	runs	correct
connection, declared then populated	:ready	runs	correct
d/db-with, declared + populated	:building	declined	now correct

Gating on (= :ready …) instead would have excluded the hand-assembled route, whose index is complete by construction and which the existing secondary-index tests depend on — including the one that asserts the delegate actually runs.

Nothing is lost but the unsound read: in the declined case the query is still answered by the columnar path that materialises from the primary indices.

Verification

• Full suite 2554 tests / 21469 assertions / 0 failures
• Focused secondary-index namespaces 75 tests / 537 assertions / 0 failures
• New regression test covers the declaring transaction, a further transaction (where an unguarded read returned 50), and the hand-assembled route still using the delegate

Found while an agent was studying the stratum defect cluster; the remaining items there (dictionary-code aggregates, storage-time Double truncation, untransactable Boolean/UUID values) are separate and still open.