Build Spring Boot 4.0.0–4.0.7: the component is in the tag, only the content archive is missing

[amondnet]

eraFor returns undefined for 4.0.0–4.0.7, so detect-upstream-versions.ts never offers them and
fetch-upstream.ts refuses them. They sit between the two Boot eras: the 4.x component path, but no
published content archive.

What is actually missing (probed 2026-09-16)

Coordinate	HEAD
spring-boot-docs/4.0.0/…-root-aggregate-content.zip	404
spring-boot-docs/4.0.7/…-root-aggregate-content.zip	404
spring-boot-docs/4.0.8/…-root-aggregate-content.zip	200
spring-boot-docs/4.1.0/…-root-aggregate-content.zip	404 (already handled — a publication fact caught by the availability gate, not an era gap)
spring-boot-docs/4.1.1/…-root-aggregate-content.zip	200

The authored half is present and in the 4.x location:

v4.0.0  documentation/spring-boot-docs/src/docs/antora/  ->  antora.yml, modules/, nav.adoc

So this is exactly the situation that produced the synthesized era for 3.3–3.x (ADR-0004): the prose
is in the tag, the generated half is not published, and it has to be rebuilt.

Why a third era rather than widening the second

eraFor picks by range, and each era pins a component path and an assembly. 4.0.0–4.0.7 share the
4.x path with the archive era but must not take its assembly — merging an archive that does not
exist is a 404, and taking the checkout alone converts with unresolved attributes (measured for
3.5.16 in ADR-0004: 350 unresolved attributes, 837 unresolved configprop:, 326 empty
include-code:: groups).

Declared oldest first, the table becomes: 3.3.0–<4.0.0 synthesized, 4.0.0–<4.0.8 synthesized (4.x
paths), >=4.0.8 archive.

The synthesis inputs exist at v4.0.0, at moved paths

SynthesisSources field	3.3–3.x	v4.0.0
examplesPath	spring-boot-project/spring-boot-docs/src/main	documentation/spring-boot-docs/src/main
staticAttributesPath	buildSrc/…/build/antora/antora-asciidoc-attributes.properties	unchanged — confirmed present
bomBuildScriptPath	spring-boot-project/spring-boot-dependencies/build.gradle	platform/spring-boot-dependencies/build.gradle
gradlePropertiesPath	gradle.properties	unchanged
metadataArtifacts	8 spring-boot-* jars	unverified — see below

spring-boot, spring-boot-autoconfigure and spring-boot-actuator all publish a 4.0.0 jar
(HEAD 200), so the mechanism carries over.

Open questions to settle first

• Which 4.0.x jars still ship META-INF/spring-configuration-metadata.json. Boot 4 restructured
  its modules, so BOOT_3_METADATA_ARTIFACTS is not transferable. Measure it against a real 4.0.x
  jar set the way that list was measured against 3.5.16 — spring-boot-test was excluded there
  because it publishes a jar and no metadata file.
• Whether the attributes properties file kept its shape across the 4.x restructure, since
  antora-attributes.ts parses it.
• Whether the BOM script is still readable by bom-libraries.ts at platform/.

Work

• measure the 4.0.x metadata-jar set
• add the third era to PROJECTS.boot.eras with the 4.x synthesis paths
• extend tests/unit/upstream-sources.test.ts (era selection at 4.0.0, 4.0.7, 4.0.8)
• convert 4.0.0 and 4.0.7 end to end under --strict, and compare the unresolved-attribute
  count against the 4.0.8 archive build
• release the eight versions and record them in catalog.json
• update the support table in README.md and the release-pipeline skill

Done when

bun run scripts/detect-upstream-versions.ts --project boot reports 4.0.0–4.0.7 as buildable, and
the catalog carries them.