feat: add data-aware task routing

[k82cn]

Summary

• add set-based task affinity and executor attribute publication over existing completion paths
• route pending tasks by affinity intersection and expose options through Python, Rust, and Runner APIs
• add vLLM Runner example, design, and data-aware Runner E2E coverage

Verification

• cargo test -p flame-session-manager --no-run
• cargo test -p flame-session-manager task_options_reject_invalid_affinity_keys
• cargo test -p flame-session-manager storage::task_tests
• cargo test -p flame-executor-manager
• cargo test -p flame-rs --lib
• focused Python core/service tests