maven-central-cache, then bilt-maven

[kenkania]

Configure Maven resolution to prefer maven-central-cache before bilt-maven in run_test.sh to align with the PR goal “maven-central-cache, then bilt-maven”

• Update run_test.sh to always write settings.xml, consistently pass -Dmaven.repo.local, and configure two explicit repositories and pluginRepositories: central2 at artifactregistry://us-maven.pkg.dev/single-scholar-280421/maven-central-cache and bilt-maven at artifactregistry://us-maven.pkg.dev/single-scholar-280421/bilt-maven. The script now honors REPO_URL via environment override with default ${REPO_URL:-artifactregistry://us-maven.pkg.dev/single-scholar-280421/bilt-private-and-maven-central} and keeps custom wagon build and .mvn/extensions.xml writing gated by REPO_URL. See run_test.sh.
• Add README_bilt.md documenting Bilt AR Wagon usage, performance notes, interface flags, and future improvements.
• Introduce a test POM at tests/bilt-1-artifact/pom.xml declaring dependency com.biltcard.common:egress-proxy:1.1.2 with Java 21 compiler configuration.

📍Where to Start

Start with repository configuration and command assembly in run_test.sh, focusing on the SETTINGS_XML heredoc and the run_test function where settings.xml is generated and Maven arguments are constructed.

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Macroscope summarized ce8f695.