Fix modello velocity phase for concurrent builder compatibility

[gnodet]

Summary

• Split modello plugin executions in maven-api-settings and maven-api-toolchain so the velocity goal (Java source generation) runs at generate-sources instead of generate-resources, while xdoc/xsd (documentation) remain at generate-resources
• Fixes a race condition in the concurrent builder (-b concurrent) where COMPILE could start before velocity finished generating Java source files, causing "package does not exist" compilation errors
• Aligns with the pattern already used by maven-api-model

Details

In Maven 4's graph lifecycle model, SOURCES and RESOURCES are parallel siblings under BUILD, and COMPILE is ordered after(SOURCES) but not after(RESOURCES):

phase(BUILD,
    phase(SOURCES),
    phase(RESOURCES),                                            ← parallel with SOURCES
    phase(COMPILE, after(SOURCES), dependencies(..., READY)),    ← waits for SOURCES, not RESOURCES
    phase(READY, after(COMPILE), after(RESOURCES)),
    phase(PACKAGE, after(READY), ...))

The modello velocity goal generates Java source files, but was bound to generate-resources (→ RESOURCES phase). This created a race: COMPILE could start while velocity was still running, missing the generated sources.

The race manifested deterministically as mvn build -T1C -b concurrent failing without clean (upstream incremental builds finished fast → COMPILE started before modello), while mvn clean build -T1C -b concurrent worked (upstream full rebuilds took longer → modello finished first).

Test plan

• mvn clean build -T1C -b concurrent -DskipTests → BUILD SUCCESS
• mvn build -T1C -b concurrent -DskipTests (without clean) → BUILD SUCCESS (previously failed with package org.apache.maven.api.settings does not exist)

🤖 Generated with Claude Code

[gnodet]

LGTM. Correct and minimal fix for a real concurrency race condition in the Maven 4 concurrent builder.

The lifecycle definition confirms the issue: SOURCES and RESOURCES are parallel siblings under BUILD, and COMPILE has after(SOURCES) but no dependency on RESOURCES. Moving the velocity goal from generate-resources (RESOURCES phase) to generate-sources (SOURCES phase) ensures COMPILE waits for the generated Java files.

Verified that all other modules in the repository already use generate-sources for the velocity goal — maven-api-settings and maven-api-toolchain were the only two holdouts. The execution ID scheme is clean: existing modello ID retained for velocity, new modello-docs ID for xdoc/xsd. Plugin-level configuration is correctly inherited by both executions.

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Claude Code on behalf of gnodet