Make the periodic reconciliation test wait for the startup pass (closes #189)

TestPeriodicReconciliationAdoptsFileThatAppearsAfterStartup slept for
three eviction intervals before writing its file, so on a slow start the
startup pass could still be running and adopt the file itself, and the
test passed without a periodic pass. It now holds the test database's
only connection until the startup pass waits for it, writes the file and
lets the connection go, as TestEvictionRunsOnPeriodicSchedule does, so
only a periodic reconciliation pass can adopt the file. Test only.

Model: opus-5-5

TODO.md

@@ -31,6 +31,14 @@ P2: security: referer blacklist
 
 # Completed Steps
 
+- 2026-10-04 `TestPeriodicReconciliationAdoptsFileThatAppearsAfterStartup`
+  only passes through a periodic pass (closes #189): it slept for three
+  eviction intervals before writing its file, and a startup pass still running
+  then could adopt the file itself. It now holds the test database's only
+  connection until the startup pass waits for it after walking the empty
+  variant directory, writes the file and lets the connection go, as
+  `TestEvictionRunsOnPeriodicSchedule` does, so only a periodic reconciliation
+  pass can adopt the file. Test only.
 - 2026-10-04 `TODO.md` merges with git's union merge (closes #190): a root
   `.gitattributes`, copied from `sneak/prompts`, marks it `merge=union`, so two
   branches that each add an entry at the top of Completed Steps merge without a