Run CI for relevant workflow changes

[yschimke]

What changed

• run the network compile workflow when shared Gradle, wrapper, or Conscrypt-fetch inputs change
• run rerun-dashboard unit tests on relevant pull requests and before every live dashboard invocation
• validate Pages Python and JavaScript changes on pull requests without deploying them
• keep PR validation concurrency separate from production Pages and dashboard operations

Why

Several shared and operational files could change without receiving the checks they affect. In particular, shared Gradle changes could break network-suite compilation, the rerun-dashboard tests were not invoked by CI, and site changes had no pre-merge validation.

The full public network tests remain scheduled/on-demand, and Pages PR validation cannot deploy production state.

Validation

• python3 .github/scripts/test_job_reruns.py
• python3 -m compileall -q site/tools
• node --check for every site/assets/*.js
• YAML parsing for the three changed workflows
• git diff --check