test(diag): exercise CUDALINK_ prefix via CUDALINK_DOORBELL, not retired CUDALINK_LIB_PATH

[forkni]

Summary

CUDALINK_LIB_PATH is being retired: cuda_link resolution no longer depends on an
env var. It's replaced by a layered lookup (explicit arg → Basefolder par → APPDATA
streamdiffusion_config.json → plain import cuda_link off sys.path) implemented in
cuda_link_bootstrap.py on the StreamDiffusionTD side.

This PR updates tests/unit/test_diagnostics.py so it no longer asserts on a variable
that's going away. It swaps the CUDALINK_ prefix-allowlist coverage over to
CUDALINK_DOORBELL, which is a live, installer-persisted variable unrelated to library
path resolution, so the same allowlist behavior is still exercised without depending on
the retired var. diagnostics.py itself needed no source change — its CUDALINK_
prefix match is generic.

Companion PRs (same root-cause fix, split by repo ownership)

• fix(td): compile CUDAIPCExtension always, never raise from callbacks forkni/cuda-link#44 — CUDAIPCExtension compiles even when cuda_link fails to
  import (was the actual crash: ModuleNotFoundError: No module named 'SHMProtocol'
  raised at class-body import time, tdAttributeError on every frame thereafter).
• dotsimulate/StreamDiffusionTD#20 — cuda_link_bootstrap.py layered resolver, drops
  the CUDALINK_LIB_PATH dependency entirely.
• fix(installer): retire CUDALINK_LIB_PATH and SDTD_BASE_FOLDER_PATH StreamDiffusion-installer#6 — installer/verifier stop writing/checking
  CUDALINK_LIB_PATH via setx.

Test plan

• ./venv/Scripts/python.exe -m pytest tests/unit/test_diagnostics.py — 29 passed
• ./scripts/git/commit_enhanced.sh lint/format gate passed on the changed file
• ./scripts/git/push_validated.sh pre-push lint passed (code lint clean; Markdown
  lint skipped via --skip-md-lint — 424 pre-existing errors in unrelated files
  SESSION_LOG.md, _hf_tracing_patches.md, two READMEs, none touched by this PR)

🤖 Generated with Claude Code

[dotsimulate]

Incorporated on sdtd041_dev. The diagnostics test now exercises the retained CUDALINK_DOORBELL variable and continues checking that unrelated secrets are excluded. Closing as incorporated on the intended development branch, without merging the original beta-based PR branch. Promotion to SDTD_040_stable is held for the operator release.

Delivered source: c6cdba3.