geotiff: extract sentinels, fallback warning, and strict-mode helper into _runtime.py (#1813 step 2) #1880

Reason

xrspatial/geotiff/__init__.py (lines 82–202) holds four object() sentinels, a UserWarning subclass, the strict-mode env-var check, and the GPU-fallback warning message helper. These are leaves of the module, with no inbound calls from elsewhere in the file. They block more interesting extractions because every backend body imports them, so isolating them first lets each later PR import from one place.

Proposal

Move them to xrspatial/geotiff/_runtime.py:

• _GPU_DEPRECATED_SENTINEL, _ON_GPU_FAILURE_SENTINEL, _CRS_WKT_DEPRECATED_SENTINEL, _MISSING_SOURCES_SENTINEL
• GeoTIFFFallbackWarning
• _geotiff_strict_mode
• _gpu_fallback_warning_message
• _Y_DIM_NAMES, _X_DIM_NAMES

Re-export GeoTIFFFallbackWarning from __init__.py (it's in __all__). Re-import the rest for the entry points still inline. Sentinel object identity is preserved by Python's module cache. Add one identity-check test to make that contract explicit.

Refs: #1813.