CHANGELOG: the 3.9 wave's 42 entries in one splice, and #3989's entries back in their sections

[erikdarlingdata]

Adds the [Unreleased] entries for the 3.9 pre-release wave in one splice: 42 entries and 42 link refs, covering #3992 through #4078. The PRs merged without editing CHANGELOG.md, and their entries were collected in the wave's buffer. Each entry goes at the top of its section, newest PR first.

The splice ran at 21:30 UTC on 2026-09-23, before the 22:00 cut-off agreed with the #3898 coordinator. It has every buffered entry whose PR was merged on dev by then. The entries for #4077 and #4079 stay in the buffer for the next splice.

#3989's entries move into their sections

#3989 put 27 entries under bare Changed and Fixed lines above ### Added. The buffer names a section Fixed, the file names it ### Fixed, and the splice script did not match the two. This splice moves those entries into ### Changed and ### Fixed, below the new entries. Their text does not change, and one blank line stays under ## [Unreleased].

Entries checked for truth

Before the splice, each entry was checked against its PR's diff. The #3898 coordinator had already checked and rewritten #4064, #4065 and #4066. Eight entries were corrected:

• Custom alert rules evaluate on the Linux compose store, using the viewer login from its verdict (#3970) #4002: the viewer role that the evaluator uses was set up by The web dashboard and MCP server connect to a compose or bring-your-own store as least-privilege roles, not as its owner (#3914) #3983. MCP connects as its own role.
• Analysis passes share each 30-day baseline once per analysis hour instead of recomputing it every pass (#3941) #4011 and Deadlock re-mask keys a missed alert instead of leaving its raw hash, walks pages to its hourly budget, and a failing stage no longer blocks the rest (#4012) #4036: the numbers now match the measurements in the PRs.
• Darling and Lite's MCP server instructions drop from ~86KB/52KB to under 8KB each (#3898) #4039: an aside that Two V137 pins met the rungs that landed around them: the v62→v63 DuckDB climb asserted the ladder's top was 63 forever, and the eight-column no-reader census read #3797's null-placeholder query_capture_mode key as a reader — dev red since #3813 (V137 residue) #3821 does not support is removed.
• Log events and deadlocks read a log_line_prefix with fields between the zone and the pid (#4041) #4047: a bare five-digit %l, %x or %v can still be read as a SQLSTATE, so the entry no longer says that only %e can.
• get_collection_health's reading guide moves off tools/list (#3898) #4063, Query Store clutter and oversized-plan backlog: MCP reading guidance moves off tools/list (#3898) #4067 and Fleet-overview and server-admin tools move reading guidance off tools/list (#3898) #4071: these PRs dropped an anecdote or an issue number, so the entries now say that no guidance was lost. The claim of "five worked examples" in get_collection_health's reading guide moves off tools/list (#3898) #4063 is removed.

#4069 had no entry, so this splice adds one.

Test plan

• Every line of dev's CHANGELOG.md is still present exactly once, except the two bare section lines and two extra blank lines.
• Untouched lines keep their order, and the moved entries keep theirs.
• The inserted lines are exactly the buffer's 42 entries and 42 link refs. The line count is 3,618 - 4 + 84 = 3,698.
• Every [#N] link under [Unreleased] has a link ref.
• The staged file matches the computed file.
• I ran the plain-English checker on the new and corrected text and fixed its real hits. The long-sentence hits that remain are the house style for these entries.
• CI.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Ua31ugERL5DmhFVRtf6keQ