Add the bounded private pipe frame codec

[arasovic]

Summary

• add a private bounded frame codec for inherited background pipes
• encode exact four-byte big-endian lengths followed by raw payload bytes
• accept the 64 MiB boundary and reject larger frames before write/allocation
• preserve clean EOF, partial EOF, arbitrary I/O errors, and short-write safety
• keep JSON payloads, roles, process spawning, and public CLI behavior out of scope

Verification

• go test -race ./internal/background -run TestFrame -count=50
• go test -race ./internal/background -count=5
• mutation proofs for endianness, pre-allocation size guard, and full-write loop
• npm run verify
• go build ./...
• Windows amd64 and Linux 386 compile-only tests
• independent Pi Worker Standards and Spec reviews; one false negative-limit test fixed, no remaining findings

Closes #197

Parent: #177