Rust begin-intermission state and effect intentions

RE-839. Reproduce with python3 scripts/analyze_rust_intermission_begin.py; focused validation: cargo xtask test-group rust-intermission-begin. Machine report.

Rust matches 39 original begin-transition cases, preserving timestamp guard semantics, fallback IDs, entity-client field clear and preparation/slow/respawn/move/publication order. Native work stays typed effects; invalid counts reject before mutation.

Original executable/native runtime and complete Linux/macOS Rust/Bevy/Solari acceptance remain unverified.