ci: cancel superseded runs and isolate test services

[djwhitt]

Summary

• cancel superseded Elixir and feature-test workflow runs
• move linting and formatting into service-free jobs
• run compilation and typing separately from the integration-service-backed test job
• retain one unified coverage test job

Review focus

• job boundaries and service requirements
• stable check names for repository protections
• preservation of the coverage command and Coveralls job identity

Validation

• workflow YAML parses successfully
• the original combined draft passed all hosted checks with this job structure

Stack

• #3772 — CI cancellation and job isolation (this PR)
• #3773 — Workload-scoped checks and caches
• #3774 — Shared Supabase integration image