Default ChildProcess unconditionally pushed every raw stderr chunk into
`#stderrChunks`, so long-lived noisy subprocesses (LSP/DAP/RPC) grew OMP
memory linearly despite the 32 KiB visible tail cap.
- Allocate `#stderrChunks` only when full capture is requested at spawn.
- Decouple retention from stream exposure via `spawnInternal`, so
`exec({ stderr: "full" })` retains without an unused live tee.
- Reject retroactive `wait({ stderr: "full" })` on a default child with a
clear error instead of returning truncated data.
Fixes#5759
roboomp
edba577e7a