feat(manifest): give each JVM build project its own id and facts component

[Jeppe Fredsgaard Blaabjerg (jfblaa)]

LLM Description written by Claude Code:claude-opus-5-5

Third step of the #1587 stack: every build project gets an identity of its own in the facts file, so projects that share a coordinate or a directory stay apart.

• projects[].id is the build tool's unique project identifier: Maven GAV, Gradle project path, sbt project id. The Maven extension keys modules by GAV, so two modules in one directory (<module>x/a.xml</module>, <module>x/b.xml</module>) are no longer merged.
• projects[].manifestFiles lists each project's own build files.
• A dependency on one of the build's own projects is that project's component, with the same id. It is linked through each tool's own resolution (Gradle's project identifier, Maven's reactor GAV, sbt's dependsOn). Gradle :a:util and :b:util, both ex:util:1, no longer collapse into one component.
• The compute-artifacts sidecar keys paths and classpaths by id internally. Its serialized shape is unchanged apart from the new project fields.

Coana compatibility: can merge independently. Released Coana (15.12.2) drops the unknown project fields, keys classpaths by subprojectDir, and keys a sibling component with no ext as jar, so it reads this output as before. Coana reads the ids from coana-tech/coana-package-manager#2547 on.

Stack: #1589 → #1590 → this → #1587

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