Regroup datasourcex reactive modules into reactive/datasource/*

[rossdanderson]

Parent

#65

What to build

Regroup the existing reactive modules physically into reactive/datasource/*, so both SDK families
will sit side by side under reactive/ once StreamLink is imported. Move physical folders only —
keep every Gradle project path unchanged by giving each module a projectDir override in
settings.gradle.kts, so published coordinates stay pinned and no cross-module project(":reactive:…")
reference changes. The five modules (reactive/core, api, kotlin, java-flow, reactivestreams)
move under reactive/datasource/; their .api snapshot files move with them.

This is a pure structural prefactor: nothing about the public API or behaviour changes.

Acceptance criteria

• The five datasourcex reactive modules live under reactive/datasource/ on disk.
• Gradle project paths and published coordinates are unchanged (datasourcex-kotlin, datasourcex-reactive-core, etc.).
• ./gradlew check passes and apiCheck reports no .api diff.
• The .api snapshot files sit alongside their relocated modules.
• CI test-report path globs (.github/workflows/build.yml) point at the new reactive/datasource/<variant> locations.
• README / AGENTS.md module-layout sections describe the new tree.

Blocked by

• None - can start immediately

[rossdanderson]

Done in commit 92b367a on branch sl4jx-consolidation. Five reactive modules relocated to reactive/datasource/* via projectDir overrides; paths/coordinates/.api unchanged; CI globs and AGENTS.md updated; check + apiCheck green.