Snail Mode game mode (BS-cb89aee1)

[byte-the-bot]

What

Implements Snail Mode: snakes leave a decaying trail of stacked hazards behind their tail as they move. Ported from the canonical Go community map (maps/snail_mode.go, authored by coreyja and jlafayette) and verified against the real Go source (no Go test file exists — the implementation is the spec, so this PR ships thorough coverage of its own).

Each turn, after the standard pipeline (move / health / hazard damage / feed / eliminate):

1. Decay — every on-board hazard stack loses one entry. Stacking = repeated Points in board.hazards; standard::damage_hazards already applies damage per entry, so a fresh length-N trail square deals N × hazardDamagePerTurn (14, the Go rules default for map-driven hazards).
2. Store — each live snake's tail square is recorded for next turn with stack = snake length, unless the tail is doubled (just ate / spawn-stacked: the tail isn't vacating). Eliminated snakes record nothing.
3. Restore — the previous turn's recorded tails land as on-board stacks, skipping (dropping) squares occupied by a live snake's head.

Food spawns normally, and food on a trail square negates that square's hazard damage (already handled in standard::damage_hazards).

State-representation decision

Pending tails are stored Go-style, as off-board points (y + board.height) inside board.hazards, rather than in a dedicated side-channel field. Reasons:

• It keeps BoardState fully self-describing, so rules::snail::execute_turn(board, moves, settings) needs no extra mutable state parameter and the trail state flows through every game loop (random runner, live apply_turn loop) for free.
• It matches Go semantics exactly, including subtle interactions (pending points are present in hazards during the damage phase, where they can only ever match a head that is itself out of bounds and about to be eliminated — same as Go).
• Persistence check: run_game holds EngineGame in memory for the entire game and never reconstructs board state from persisted frames — a retried GameRunnerJob on a Running game calls reset_game_state_for_retry (wipes turns/placements) and replays from turn 0, and a Finished game short-circuits to idempotent post-completion hooks. So pending-tail state does NOT need to round-trip through frame persistence, and frames stay a write-only feed for the board viewer.

Both serialization boundaries filter through the new BoardState::on_board_hazards() view:

• wire.rs (/move payloads) — snakes never receive out-of-bounds hazard points (hard requirement; test-enforced).
• frame.rs (persisted frames / board viewer) — frames carry only real on-board hazards, stacked duplicates included, matching what play.battlesnake.com showed. Off-board bookkeeping points are excluded from frames because nothing reads frames back into engine state and they would only confuse the viewer and any other frame consumer.

Wire-protocol parity (game.map)

On play.battlesnake.com, snail was a map, not a ruleset: snakes saw ruleset.name = "standard" with game.map = "snail_mode". Arena's wire layer already had a map field (always empty). This PR keeps that upstream parity: internally the engine dispatches on meta.ruleset_name == "snail_mode", but the wire sends ruleset.name = "standard" + game.map = "snail_mode", so existing community snakes that key off game.map behave correctly. Other modes are unchanged (map stays "", ruleset name passes through).

Deviations from Go (documented in module docs)

• Go's doubleTail would panic on a 1-segment snake; this port treats bodies shorter than 2 as doubled (no trail). Unreachable in real games.
• Go rebuilds hazards via a map-editor with nondeterministic map-iteration order in the decay phase; this port keeps deterministic first-seen order. Hazard multisets are identical.
• Go's StandardMap.PostUpdateBoard also spawns food; the arena engine spawns food separately (same as every other mode here), so the snail post-update is hazards-only.

Tests

• rules::snail unit tests: trail appears with stack = snake length one turn after recording; decay by exactly 1/turn/square until gone; double-tail (just ate / spawn-stacked) spawns no trail; eliminated snakes leave no new trail (but their previously recorded trail still lands); head-occupied squares skip placement and the stack is dropped, not deferred; multiple snakes' trails stack independently; per-stack-entry damage (3×14 on a fresh length-3 square) and lethal-stack elimination cause; food negates trail damage; early game-over exit.
• Worked 5-turn example from the port analysis as an integration-style test (exact on-board multisets per turn).
• Property tests (proptest, no hardcoded with_cases, no committed seed files): an independent per-square oracle (new = max(old−1,0) + restored·(no live head)), off-board entries exactly equal live non-double-tail snakes' tails × length, full decay without snakes, determinism, and no malformed hazard entries.
• Server tests: create_initial_game snail settings; live-loop-style frame test (trails appear in frames, off-board points never leak); random-runner smoke test; wire parity tests (standard + snail_mode map, other modes unchanged, off-board filter).

cargo fmt --all, cargo clippy --workspace --all-targets (clean), cargo test -p rules (91 passed), full cargo test -p arena --bin arena (420 passed) all green.

Merge note

This PR trivially conflicts with the parallel Constrictor PR in the server/src/engine/mod.rs dispatch matches (create_initial_game and the per-turn match game.meta.ruleset_name.as_str()): both PRs make the identical refactor and each adds its own arm, so the resolution is a mechanical union of arms.

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