devEngines.packageManager PR Plan

This plan breaks the devEngines.packageManager work from SPEC.md into reviewable PRs.

PR 1: Rust support for devEngines.packageManager

• Add structured PackageJson.dev_engines support.
• Preserve unknown and ignored devEngines fields during serialization.
• Implement Rust detection precedence: top-level packageManager, then devEngines.packageManager, then missing-declaration error.
• Validate supported names, exact semver versions, malformed objects, whitespace, and empty values.
• Reuse existing pnpm and Yarn version mapping.
• Add lockfile mismatch validation.
• Wire dangerouslyDisablePackageManagerCheck to bypass declaration validation and use existing implicit detection behavior.
• Update Rust diagnostics and messaging to recommend devEngines.packageManager.
• Add Rust unit coverage and representative CLI integration coverage.

PR 2: TypeScript detection parity

• Update @turbo/workspaces package-manager detection to support devEngines.packageManager.
• Match Rust precedence, validation, exact-semver-only behavior, array rejection, malformed-object handling, and ignored unknown properties.
• Update TypeScript user-facing errors to prefer devEngines.packageManager.
• Add TypeScript detection tests.

PR 3: Codemod update

• Keep the add-package-manager transformer name.
• Write devEngines.packageManager instead of top-level packageManager.
• Treat existing top-level packageManager as already set.
• Treat existing devEngines.packageManager as already set.
• Shallow-merge into existing devEngines.
• Update codemod description and logging.
• Add tests for output shape, merge behavior, and idempotency for both declaration fields.

PR 4: Workspace conversion tooling

• Update add, remove, and convert behavior to handle both declaration fields.
• Remove stale top-level packageManager when converting to a new manager so precedence cannot keep resolving to the old manager.
• Preserve either declaration field when it identifies a different manager than the one being removed.
• Add tests for add, remove, convert, stale top-level removal, and preservation behavior.

PR 5: Docs and user-facing copy

• Update docs, errors, comments, and generated schema descriptions to prefer devEngines.packageManager.
• Mention top-level packageManager only as legacy or backward-compatible support where useful.
• Update dangerouslyDisablePackageManagerCheck descriptions to mention both declaration fields or use declaration-neutral wording.

PR 6: Examples, templates, and scaffolding

• Update examples, templates, and new project scaffolding to write devEngines.packageManager.
• This PR should happen after a stable release includes devEngines.packageManager detection support, so generated projects remain compatible with currently supported Turborepo versions.