Hosted Maven and VEX treat a Maven 4 implicit-subproject root as one pom #1809

[agent] Split out of the review of #1807 (the vendored fix for #622).

Summary

Maven 4 discovers the subprojects of a 4.1.0 pom-packaged root that lists no <subprojects>/<modules>: every direct subdirectory holding a pom.xml (checked against Maven 4.0.0-rc-7's DefaultModelBuilder and a real mvn validate). #1807 teaches the vendored reactor planner this. Every other caller still decides "is this a reactor?" with maven_reactor::declares_modules, which only sees declared lists:

• patch/redirect/mod.rs (rewrite_maven_pom): the redirect_maven_multimodule_unsupported refusal (Hosted Maven mode edits only the root pom of a multi-module build, leaves child literals unpatched, and still attests VEX #261) doesn't fire. Hosted mode may pin only the root pom while a discovered module's literal <version> wins.
• vex/discover/maven.rs:158: reactor is false, so the root's hosted pin can be attested instead of being reported UnattestedKind::MavenReactorRoot.
• vendor/jvm/sbt.rs:261: the vendor_jvm_build_ambiguous guard next to build.sbt is skipped. sbt.rs:98 and jvm/mod.rs:350 (MavenShape) also classify the root as a single pom.
• crawlers/maven_scope.rs: the project-mode scope seeds only declared modules.

Expected

The same discovery rule (model version not 4.0.0, literal pom packaging, no top-level list, maven.project.discoverSubprojects not set to anything but true) decides reactor-ness everywhere. The hosted callers need the child directories in their file view to apply it.

Not verified

Code-path analysis only. No hosted repro has been run yet. The first step is a failing hosted test (scan --mode hosted + vex on the #622 tree).

Generated by Claude Code