Release headings are append-only: the ceremony (#111) adds one and nothing
in CONTRIBUTING's release flow ever removes one. Nothing asserted that.
The arming rule (test/release.test.ts, rig#66) is narrow by design — it asks
whether the TOP section agrees with package.json's version, about ONE
heading, the one a PR is about to write under. It says nothing about the rest
of the file, and cannot: "a heading disappeared" is not a property of a tree,
it is a property of a DIFF.
So an author adding an entry under '## Unreleased' who types OVER the heading
below it instead of inserting above it produces a tree every existing guard
calls green. git merges it cleanly — a one-line edit in a file nobody touched
concurrently, no conflict, no signal. The shipped section's body is now
sitting under '## Unreleased' and the version it belonged to has no section
at all. It surfaces at the NEXT release, when release-notes.sh cannot find
the section it extracts by heading, or worse republishes the absorbed prose.
Ports box's changelog-monotonic.sh (box#122, caught in review of box#118)
rather than reimplementing the invariant a third time in TypeScript, and
keeps both halves. Containment catches a DELETED heading; it cannot catch a
DUPLICATED one, because a duplicate is head-side surplus and base-minus-head
is blind to extras on the head side. Uniqueness on HEAD is asserted alongside
it, and that half matters more in cast than in box: release-notes.sh's awk
has no `exit`, so `grab` re-arms on every matching '## ' line and two copies
of a version heading make the published body ABSORB whatever sits between
them — with the stranded entry dropped from the next release's notes too.
(rig's extractor truncates instead; cast has the absorbing one.) The existing
"double re-arm" test covers duplicate '## Unreleased' only, not duplicate
VERSION headings, which are the ones that reach release-notes.sh.
Wired into ci.yml as its own step so a red run names the invariant that
broke; pull requests only, because on a push to main the merge base IS HEAD
and the assert is vacuous; STRICT=1 with fetch-depth: 0 so a checkout that
cannot reach the base ref fails loudly instead of skipping quietly forever.
'## Unreleased' stays outside the guarded set — the arming rule owns that
heading and the ceremony legitimately consumes it.
Closes#133
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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