docs: validate recommended deployment snippets

[matthewygf]

Problem

PR #285 added two deterministic command failures to the recommended deployment guides:

1. The DSv4 P2P startup script enables set -euo pipefail and then expands $LD_PRELOAD directly. On a clean host/container where the variable is unset, Bash exits with LD_PRELOAD: unbound variable before LMCache or vLLM starts.
2. The GLM-5.2 benchmark command requests v0.4.4· (with a trailing Unicode middle dot) instead of the existing upstream v0.4.4 tag.

The first failure is the same shell pattern addressed for the dsv4_support_045 documentation in #279, but #279 targets a different base branch and different file paths, so it cannot repair the new main guide.

Fix

• Preserve an existing preload list with Bash's nounset-safe conditional expansion.
• Correct the LMCache tag to v0.4.4.
• Add documentation regression tests that scan recommended shell snippets for unsafe LD_PRELOAD expansion under nounset and reject non-ASCII/unsafe documented clone refs.

Validation

• Reproduced the original unset-variable command: exit 127 with LD_PRELOAD: unbound variable.
• Corrected expansion passes with LD_PRELOAD both unset and pre-populated.
• Confirmed upstream refs/tags/v0.4.4 exists with git ls-remote.
• Both new regression functions pass when invoked directly.
• python -m py_compile tests/test_documentation_snippets.py passes.
• SPDX header check passes.
• Repository-pinned isort 6.0.1, ruff 0.11.7, and ruff-format checks pass.
• git diff --check origin/main passes.
• GitHub Code Quality passes.

Risk

Low. This changes two documentation commands and adds standard-library-only tests; no runtime code is modified.