Give each Zed Cargo layer its own plan

[gauravtiwari]

Summary

• compare cold, target-only, sccache-only, and combined Cargo layers through committed .boringcache.toml plans
• keep the GitHub Action as the orchestrator while the released CLI owns cache identities, target selection, compiler-cache selection, and Cargo commands
• preserve the existing rolling cache identity without workflow-time tag mutation
• retain native Cargo/sccache evidence for each primary and remote-server phase

The reviewed source pair is 4e8057d74db3570b3bd419ff296eb84c35b3a5a3 → d4010e91ccc9635853698af160f15bb479a484d0. It is one adjacent upstream commit changing only project-panel undo/trash behavior and tests; Cargo metadata, lockfiles, and the Rust toolchain are unchanged.

Validation

• python3 -m unittest discover -s test -v
• python3 scripts/verify-zed-release-recipe.py upstream
• released CLI 1.18.1 dry-run for all four primary plans
• YAML parse and git diff --check

[gauravtiwari]

The committed Cargo layer plans are already on main. The remaining replay correctness fixes and a fresh source-scoped proof cohort are isolated in #14, so this duplicate draft is superseded.