task(ci): optimize PR builds further

[ndizazzo]

Summary

This PR reduces pull request CI time and improves CI reliability by splitting the slow Linux/macOS CPU artifact path from test fanout, making Rust cache restore failures non-fatal, avoiding unnecessary Windows Node release addon builds, and removing Docker image builds from PR workflows.

What changed

• Added early Linux and macOS CPU artifact producer jobs so downstream smoke and SDK jobs can start sooner instead of waiting behind serialized test work.
• Split Linux SDK/API, Skippy, unit, protocol, and Skippy smoke coverage into parallel test-group jobs.
• Wired Linux Cargo jobs through RUSTC_WRAPPER=sccache and added sccache stats reporting so cache usage is visible in logs.
• Made Swatinem/rust-cache restore failures non-fatal in PR workflows; cache restore outages now degrade to cold builds instead of failing required checks.
• Added Docker Buildx cache initially, then removed PR Docker builds entirely because Docker validation is moving outside this repo’s PR CI.
• Gated the expensive Windows Node SDK release addon build behind direct release-relevant inputs while keeping the fast Windows Node SDK cargo check coverage.
• Updated CI documentation in ci/ci.md to match the new PR workflow topology and Docker ownership.
• Cleaned up workflow lint issues found by actionlint.

Performance notes

• Baseline failed PR Builds run 26383941773: 32.80m wall clock, with Windows CUDA failing during Rust cache restore.
• Cache-resilience run 26384942371: passed, and Windows CUDA completed in 1.27m, but total wall clock was 35.07m because Windows CPU/ROCm became the critical path.
• Windows Node gating run 26386317939: passed; Windows CPU execution improved from 34.82m to 22.40m by skipping the duplicate Node release addon build on workflow-only changes. Wall clock was still 36.68m due to runner queue time.
• Removing PR Docker builds eliminates a separate PR Docker check that recently cost about 18.5m on this branch.

Validation

• actionlint
• git diff --check
• Workflow/action YAML parse validation
• Duplicate workflow step ID validation
• PR trigger policy validation
• LSP diagnostics for .github/workflows
• cargo run -p xtask -- repo-consistency release-targets
• cargo run -p xtask -- repo-consistency ci-crate-lists

Notes

The latest PR head after Docker removal schedules only PR Builds and PR Quality Checks; no new PR Docker Build run was created for that head.

[i386]

I super appreciate this work @ndizazzo