Release 3.9.0

[erikdarlingdata]

What this is

The 3.9.0 release, cut from dev 9e1b619e. It replaces #4922, which was cut from an earlier dev.

What changed

• The version: Directory.Build.props goes from 3.8.0 to 3.9.0.
• The CHANGELOG:
  • the entries carried by the PRs merged to dev between 2026-09-28 and 2026-09-30 are spliced into [Unreleased], with the managed store's random_page_cost entry (The managed store sets random_page_cost = 1.1, tuned for SSD storage #4863) under Changed;
  • [Unreleased] becomes ## [3.9.0] - 2026-10-02, with a new empty [Unreleased] above it;
  • an Important section: collectors on large fleets now run at their configured interval, and upgrading from 3.8 runs the store migrations V126 through V157 on the service's first start;
  • the entries of the fixes merged since the last splice are added: 45 under Fixed and 1 under Changed, with 57 link definitions;
  • two existing entries are amended ([BUG] No headless way to enable a collector: long_query_completions ships opt-in but only the WPF Viewer can opt in #3752 with provision-roles.sql stops at its role-collision guard, and the collector toggle verbs exit 2 for a store problem (#4746, #4744) #4809's exit codes, and PostgreSQL-target analysis engine: v2/v3 slices and the v1 lanes' residue (from #3542) #3691 with The lock-wait fact counts each logged line once and each lock wait once #4886's counting).
• The archive: the 3.9.0 entries move to docs/changelog/3.9.md, and tools/changelog/archive-census.txt is regenerated. tools/changelog/changelog_archive.py verify passes for 3.9.0 with 760 entries, and the round trip holds.
• The upgrade test's fixture: Darling/Darling.Tests/Fixtures/migration-ladder-v3.8.0.sql, the v3.8.0 store schema (124 migrations, top V125) that the migration upgrade test climbs from to V157.

Not changed

No product code or configuration.