Prove App Builder CLI failure and interruption cleanup

[kattsushi]

Part of #131. Follows #146 and PR #147 as Unit 6B of the App Builder feature-branch chain.

Problem

Unit 6A completes the successful installed public CLI proof, but failure and interruption process-tree cleanup remain intentionally unproven. A direct child may close while a TERM-resistant descendant survives if the delayed process-group SIGKILL is canceled.

Scope

• Prove a real generated Nx verification failure through the installed public CLI boundary.
• Prove real generated Nx/Vitest interruption after explicit readiness evidence.
• Run a descendant that ignores SIGTERM in the same POSIX process group.
• Retain the process-group identity (PGID) independently of the direct-child lifecycle.
• Send SIGTERM to the process group, poll for group disappearance within a bounded interval, escalate to group SIGKILL, and await confirmed disappearance.
• Assert observable escalation and stopped-process evidence.
• Clean up the isolated workspace, package store, and all spawned processes while preserving repository-root Nx isolation on macOS and Linux.

Acceptance criteria

• The behavior-first test generated Nx verification failure cleans the complete process group passes against a real generated Nx verification failure.
• The behavior-first test generated Nx/Vitest interruption escalates and removes a TERM-resistant descendant after readiness passes against a real generated interruption after readiness evidence.
• macOS and Linux CI provide the failure, interruption, escalation, stopped-process, cleanup, and root-isolation evidence.
• No test substitutes a synthetic process.exit(1) failure or timer-based interruption proxy for the generated Nx/Vitest behavior.
• The delivery stays within 400 changed lines (additions + deletions).
• Rollback removes only Unit 6B and leaves Unit 6A intact.

Non-goals

• Successful generation, packaging, naming, CRUD, or replay behavior already handled by Prove successful installed App Builder CLI generation #146 and PR feat(app-builder): prove installed CLI generation #147.
• The Windows process model.
• Unit 7 eligibility work.
• Any mutation, regeneration, retargeting, or other change to PR feat(app-builder): regenerate canonical Todo showcase #130.

Chain context

• Delivery base: feat/app-builder-public-cli-success-proof at dc97f7a09effc1159cc3bcdc92801723a2b78112.
• Chain: #143 (+ #145) → #147 Unit 6A → 📍 Unit 6B → Unit 7 → frozen #130.

Part of #131. This issue does not close #131 or #146.

[kattsushi]

Closing as not planned because Effectify App Builder has been retired in favor of contributing to lloydrichards/stack-effect. No App Builder implementation is retained on dev.