Constrictor game mode (BS-cb89aee1)

[byte-the-bot]

What

Implements the Constrictor game mode in the Rust engine. Constrictor is the standard ruleset plus two extra stages that run after elimination every turn:

• RemoveFood — all food is cleared from the board.
• GrowSnakes — every snake is pinned at health 100 and its tail segment is duplicated (grows by one), unless the tail is already stacked (tail == sub-tail).

No food ever spawns, snakes never starve, and hazards are inert. Games end via collisions and space exhaustion as ever-growing bodies fill the board.

Mapping to the Go rules

Ported from BattlesnakeOfficial/rules (constrictor.go / constrictor_test.go, verified against the live repo):

• rules/src/constrictor.rs::execute_turn mirrors constrictorRulesetStages: game-over check → move → starvation → hazard damage → feed → eliminate → remove_food (RemoveFoodConstrictor) → grow_snakes (GrowSnakesConstrictor) → turn increment.
• Matching Go exactly, grow_snakes does not skip eliminated snakes — a snake that dies this turn still ends it grown and at health 100 (asserted by the ported constrictorMoveAndCollideMAD case).
• Init semantics: the Go engine runs the full pipeline once with no moves at game start (ruleset.Execute(boardState, nil)); the standard stages no-op via IsInitialization, leaving RemoveFood + GrowSnakes. modify_initial_board reproduces that: the initial board has its food stripped and snakes at health 100; the stacked 3-segment starting bodies don't grow yet (tail == sub-tail).
• One documented deviation: Go would panic indexing body[len-2] on a 1-segment snake; the Rust port grows it instead (a lone tail is definitionally not stacked). Unreachable in real games — snakes start at 3 segments.
• Settings: foodSpawnChance: 0, minimumFood: 0, hazardDamagePerTurn: 15. The Go repo defines no constrictor-specific settings (the CLI applies its global 15/1/14 defaults to every mode); 0/0 correctly advertises "no food ever spawns" on the wire, and hazard damage is inert either way.

Engine wiring

• create_initial_game gains a GameType::Constrictor arm and applies the init modifications.
• Per-turn dispatch in both paths — run_game_with_random_moves (execute_turn) and apply_turn (the live game_runner path) — is refactored from match &game.meta.royale to match game.meta.ruleset_name.as_str() with "royale" / "constrictor" / _ arms.
• maybe_spawn_food (called outside execute_turn) is skipped for constrictor games.
• Wire protocol needed no structural change: ruleset.name = "constrictor" and the settings flow from GameMeta; a wire test locks in the shape.

Test coverage

• Ported Go cases: constrictorMoveAndCollideMAD with exact bodies, health 100, elimination causes/by/turn, food cleared; standardCaseErrNoMoveFound; standardCaseErrZeroLengthSnake.
• Unit tests (rules): food removed each turn and at init, growth + health pinning, no growth on stacked tails, eliminated snakes still grown/health-set, eating doesn't double-grow, starting-stack unwind timing, early-exit no-op, never-starve.
• Property tests (rules, default proptest config): food always cleared, health pinned at 100 for all snakes, tail stacked after every turn, growth exactly determined by feeding/pre-move tail stacking, turn increment, game-over early exit is a no-op. Properties are elimination-agnostic, so they don't trip over the two-phase elimination ordering.
• Engine tests (arena): constrictor create_initial_game settings + modified initial board, live-loop (apply_turn) frames never contain food and snakes grow, eliminated snakes still grown in the live path, full random-move constrictor games terminate, wire serialization.

Results: cargo fmt --check clean, cargo clippy --workspace --all-targets zero warnings, cargo test -p rules 90 passed, cargo test -p arena --bin arena 419 passed.

Merge note

This PR trivially conflicts with the parallel Snail Mode PR in the server/src/engine/mod.rs dispatch matches (create_initial_game, run_game_with_random_moves, apply_turn) — both add an arm to the same match game.meta.ruleset_name.as_str() blocks, so the resolution is a mechanical union of arms.

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