SW-207 — Handle child stdout and stderr stream errors in the dev supervisor #389

Summary

The supervisor subscribes to data/end events on child stdout/stderr but does not register stream error handlers. A stream read failure can become an unhandled error or leave the supervisor waiting for a process it can no longer observe. Convert stream failures into a bounded, sanitized subprocess failure and terminate siblings safely.

Workstream

• Workstream: WS-PROCESS
• Primary scope: src/dev/orchestrator.ts; tests/dev-orchestrator-startup-and-buffering.test.ts; tests/dev-supervisor-cleanup.test.ts
• Protected surface: Yes — maintainer review required
• Priority: P1
• Difficulty: high
• Proposed points: 200

Acceptance Criteria

• Observe stdout and stderr errors independently.
• A stream error rejects with a stable user-facing category.
• Sibling processes are signalled for cleanup.
• Raw stream errors and local paths are not printed.
• Simultaneous errors/exits settle the supervisor only once.

Dependencies

None unless noted above.

Out of Scope

• Unrelated architecture or public command-contract changes.
• New runtime dependencies without maintainer approval.
• Mainnet deployment or remote template execution.

Definition of Done

• Acceptance criteria have deterministic regression coverage.
• Security and compatibility boundaries remain intact.
• Supported-platform CI passes.

Labels

• Stellar Wave (do not apply until activation is explicitly authorized)
• type: test
• area: process
• priority: P1
• difficulty: high
• points: 200
• workstream: process

Sequencing and scope note

Sequencing: implement after SW-206 (#388) so stream errors settle through the established close/finalization path.