docs: make DSv4 startup scripts safe with unset LD_PRELOAD

[matthewygf]

Problem

The DSv4 guides start their launch scripts with set -euo pipefail, then expand $LD_PRELOAD directly:

export LD_PRELOAD=/usr/lib/aarch64-linux-gnu/libjemalloc.so.2:$LD_PRELOAD

On a host/container where LD_PRELOAD is unset, Bash exits with LD_PRELOAD: unbound variable before the LMCache controller or vLLM server starts. The pattern appears in the new cross-node P2P guide and both launch examples in the existing DSv4 guide.

Fix

Use Bash's conditional expansion in all three snippets so jemalloc is set when the variable is absent and any existing preload list is preserved when present.

Validation

• Unset-variable case under bash -u produces exactly the jemalloc path.
• Existing-value case preserves /existing.so after jemalloc.
• Repository search confirms no unsafe DSv4 LD_PRELOAD=...:$LD_PRELOAD snippet remains.
• git diff --check passes.

Risk

Documentation-only, three-line shell correction. It does not change runtime code or the intended jemalloc preload order.