CHANGELOG: the 2026-09-18 board - fifty-four entries from two seats, spliced once

Fifty-nine PRs merged to dev today across the coordinator's lanes and the wave-2
worker's; each lane returned its entry to a buffer instead of touching this file,
so that fifty-plus PRs did not each rebase the same twenty lines. This is the one
splice. Every entry is one line (the archiver's compact() and the pins read them
that way); riders fold into their parent's entry (#3599 under #3590, #3619 under
#3611, #3623 under #3616, #3640 under #3633; #3617 test-only and #3661 re-cut as
#3666 carry none); #3657's entry is in because it MERGED to dev - the twin to main
is what is still pending.

[Unreleased] gains a `### Added` above `### Fixed` (Keep-a-Changelog order) for the
six new capabilities: per-user theme colours (#3606 / #3577 arm B), routed alert
families (#3668 / #3598), the PostgreSQL logging audit tool (#3643 / #3607), the
service-side wait sampler (#3645 / #3604), and the log-event classifier with its
temp-file / autovacuum parser families (#3646 / #3601, #3664 / #3602 #3603). The
other forty-eight are honesty fixes to existing surfaces and append to `### Fixed`
after the wave-1 bullets, in PR-number order.

Thirty-six reference definitions added for the issues the new entries cite and the
index did not yet define; the [Unreleased] group is one ascending run again, which
moves [#3557] into its slot (the one deleted line). Nothing under ## [3.8.0] or
older is touched; the archive script was not run. One editorial touch: the #3585
entry ended in a dangling "Darling" and now reads "Darling only." (the tool exists
only in the Darling MCP host).

tools/changelog/changelog_archive.py verify: all PASS (1420 bold entries, floor
1,329; 1,358 distinct refs resolve; CRLF throughout; 356,539 bytes under the
750 KiB ceiling). ChangelogIndexAndArchiveTests: 5/5 pass via a net10.0 harness.

Author: erikdarlingdata