Examples: Standardize naming conventions #1675

Parent issue: #1673

Goal

Decide on and apply consistent variable names for common objects across all examples.

Current state

• Stream variables: s vs stream vs stream0/stream1
• Kernel variables: ker vs add_kernel vs gpu_ker
• Device variables: dev vs dev0/dev1 vs inline Device()
• Launch config: mostly config, but multi-config uses config0/config1

Decisions needed

1. What should the standard name be for each common object type?
2. What suffix convention for numbered variants (multi-GPU, multi-stream)?
3. Should these conventions be documented somewhere for future contributors?

Files affected

All 25 example files listed in #1673.