Model pnpm dev installs by overlap-connected workspace component

Problem

With the current source-linked workspace:* development model, pnpm:install:<pkg> is not a real package-local isolation boundary.

When per-package pnpm-workspace.yaml files overlap, pnpm install mutates node_modules for the full overlap-connected workspace component, not just for the package whose task was invoked. That makes the current task model misleading and causes timing-sensitive behavior in dev.

Concrete symptom:

• #254: tests can fail because a later pnpm:install:<other> task rewrites node_modules in an overlapping package while test:<pkg> is running

Important scope split:

• #333 addressed the Nix CLI build path via staged closure + pnpm deploy
• the remaining problem is the dev install/task boundary, not the build path

Root cause

The real mutable boundary is not a package. It is the set of packages whose install write-sets overlap.

For a package P, define:

• W(P) = {P} ∪ workspaceMembers(P)

If W(P) and W(Q) overlap, installs for P and Q can rewrite the same node_modules state and therefore are not independent.

Goal

Change the dev install abstraction from package-scoped to overlap-connected workspace-component-scoped, so the task graph matches the actual isolation boundary.

Desired shape

• Compute overlap-connected components from workspace-member write-sets.
• Make install tasks operate on the component, not pretend to be package-local.
• Make test:<pkg> depend on the install boundary for the component that contains <pkg>.
• Preserve parallelism across disjoint components.
• Use deterministic ordering inside a component where needed.

Acceptance criteria

• test:<pkg> remains stable without requiring a global install barrier.
• No task runs while another install can still mutate the same overlap component.
• The task graph reflects the actual pnpm side-effect boundary.
• Source-linked workspace:* development semantics are preserved.
• Parallelism is preserved across disjoint components.

Non-goals

• Do not assume true per-package install isolation is achievable under the current source-linked workspace:* dev model.
• Do not conflate this with the Nix build-path work already shipped in #333.

Related

• Supersedes #254 as the canonical remaining dev-path issue
• Follow-up to #322
• #333 solved the separate Nix build-path isolation problem

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_{Edited by an AI assistant on behalf of @schickling}

Start by tracing the dev install task graph and the per-package pnpm-workspace.yaml files to identify workspace-member write-set overlaps. Review the distinction from #333, then model overlap-connected components and verify that tests in each component cannot run during conflicting installs while disjoint components remain parallel. Done means the stated acceptance criteria hold without changing source-linked workspace:* development semantics.