Add the hidden same-binary role process boundary

[arasovic]

Summary

• add fixed private supervisor/worker-host role tokens
• launch the same executable with fd3 request and fd4 response pipes while keeping payloads out of argv/env/stdout/stderr
• add bounded framed Send/Receive and exact request EOF handling
• add concurrent-safe Wait, Kill, and Close that reap the exact child without process-group claims
• mark child role fds close-on-exec so future Pi descendants cannot mask EOF
• keep production role dispatch, JSON payloads, ownership-loss handling, Pi, Manager, and CLI out of scope

Verification

• real same-binary subprocess echo with binary and 1 MiB payloads
• blocked Send and blocked Receive cleanup; early child exit; oversized frame; repeated start failure fd audit
• go test -race ./internal/background -run TestRoleProcess -count=20
• go test -race ./internal/background -count=2
• mutation proofs for ExtraFiles order, parent duplicate response-end closure, close-on-exec, and Kill-before-request-close ordering
• npm run verify
• go build ./...
• Windows compile-only background package test
• independent Pi Worker Standards and Spec reviews; findings resolved or disproved with subprocess regressions

Closes #199

Parent: #177