[HYDRA-1950] Crash when unloading/reloading Maya Hydra plugin

**Describe the bug**
This crash has been at least occurring for all of the 2026 releases (including 2026.2, but probably also in earlier releases).
Crash occurs when loading the Maya Hydra plugin, unloading, and then reloading the plugin.

Seems to be caused by the TF_DEBUG symbols being registered multiple times due to the re-load.

**Traceback**
```
------------------------------- Maya terminated --------------------------------
Maya crashed. FATAL ERROR: [TF_DEBUG_ENVIRONMENT_SYMBOL] multiple debug symbol definitions for 'MAYAHYDRALIB_RENDEROVERRIDE_DEFAULT_LIGHTING'. This is usually due to software misconfiguration, such as multiple versions of the same shared library loaded simultaneously in the process. Please check your build configuration.
in _Register at line 122 of /Volumes/DATA/local/S/jenkins/workspace/ECP/ecg-usd-build/ecg-usd-full-python3.11-osx/ecg-usd-build/usd/pxr/base/tf/debug.cpp
writing crash report to [ theta.local:/var/folders/8k/kd1nc9d904n1hpp457tbmh180000gn/T//st_Maya.46264 ] ... done.
--------------------------------------------------------------------------------
zsh: abort Maya
```

**Steps to reproduce**
Steps to reproduce the behavior:
1. Load Maya
2. Open the Plug-in Manager and load the Maya Hydra plugin (mayaHydra.bundle)
3. Unload the Maya Hydra plugin
4. Reload the Maya Hydra plugin again
5. Crash

**Expected behavior**
Maya should not hard-crash and potentially result in lost work.

**Specs (if applicable):**
- MacOS 26
- Using the Maya Hydra bundled with Maya 2026/2026.2
- Using the default USD 24.11 config

Reproduce the failure by loading, unloading, and reloading mayaHydra.bundle in Maya 2026. Start with the TF_DEBUG symbol registration failure reported at usd/pxr/base/tf/debug.cpp in _Register. Done means the plugin can be reloaded without duplicate-symbol errors, a hard crash, or lost work.

Written by the indexing model from the issue text.