CI: Enhance workflow reliability and consistency

[jserv]

This improves GitHub Actions workflows:

• Add reliable-download.sh wrapper with automatic retry logic and atomic file operations for all network downloads
• Add concurrency control to prevent duplicate workflow runs
• Improve error handling in test loops with explicit failure messages
• Replace inconsistent wget/curl calls with centralized wrapper
• Increase network timeout values (30s connect, 60s read)
• Download wrapper prefers curl, falls back to wget
• Implements atomic downloads with PID-based temp files
• Adds command existence validation before execution
• Includes trap handlers for cleanup on cancellation

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Summary by cubic

Improves CI reliability and consistency by adding a robust download wrapper and enforcing workflow concurrency. Reduces flaky network steps, duplicate runs, and noisy failures across jobs.

• New Features

  • Added .ci/fetch.sh with automatic retries, 30s connect and 60s read timeouts, and atomic writes.
  • Prefers curl and falls back to wget; validates command availability and cleans partial files via traps.
  • Clear logging and quiet mode for predictable output.

• Refactors

  • Replaced all ad-hoc wget/curl calls with the wrapper.
  • Added concurrency groups with cancel-in-progress to prevent duplicate runs; deployments avoid cancellation.
  • Consolidated build/test variants into loops with explicit failure messages.
  • Added caching for LLVM/toolchain and build artifacts to cut network requests.
  • Standardized runners to ubuntu-24.04 and set job timeouts across workflows.
  • Disabled JIT in emcc builds to stabilize wasm jobs.

^{Written for commit d85d0ed. Summary will update automatically on new commits.}