Examples: Standardize imports and dependency policy

[rparolin]

Parent issue: #1673

Goal

Apply consistent import ordering and evaluate external dependency usage across all examples.

Current state

• At least 5 different import ordering patterns exist across the 11 cuda.core examples.
• Some start with sys, some have no stdlib imports, some use try/except for optional deps inline.
• No consistent grouping (stdlib → third-party → cuda).
• CuPy is required by 7 of 11 cuda.core examples.
• Only show_device_properties.py has zero external dependencies.
• Optional dependency handling (try/except) is inconsistent.

Decisions needed

1. What import group ordering should examples follow (e.g., stdlib → third-party → cuda)?
2. Should we reduce CuPy dependency where possible (e.g., use NumPy + manual device allocation)?
3. What is the standard pattern for optional dependency handling (try/except with skip vs hard fail)?

Files affected

All 25 example files listed in #1673.

[cpcloud]

Feels like a thing 100% solved by making the ruff config global and letting it run on examples.