engine: attack and block requirements, and blocking an additional creature

[ninthworld]

Parent: #706. One of the exclusion families blocking the 104 M19 cards the catalog does not yet define.

Exclusion (crates/sage-engine/data/exclusions.json): Attack and block requirements, and blocking an additional creature — a declaration can be restricted but never required, and a blocker blocks one attacker.

M19 cards blocked (2)

#	Card	Shape
181	Ghastbark Twins	may block one additional creature each combat — otherwise a vanilla trample body
175	Declare Dominance	every creature able to block the pumped creature must do so

Scope

Two independent changes, and the first is much cheaper than the second:

1. A blocker blocking more than one attacker. Today the declaration maps one blocker to one attacker. Lifting it touches the block legality check, the combat damage assignment (a blocker assigns its damage among the attackers it blocks), and the view. Ghastbark Twins is a permission, not a requirement, so it needs no maximisation logic at all.
2. Requirements. CR 509.1c is the hard one: the declaration must satisfy the maximum possible number of requirements without violating any restriction, which makes validating a submitted block a search rather than a per-pair check. Restrictions still win over requirements, and a requirement that cannot be met is simply not met.

Do them in that order, in separate PRs. Requirements also need a clear answer for what the client is told — the blocker slot's prompt already carries whole-declaration facts like menace, and a requirement is another one.

Acceptance criteria

• Ghastbark Twins authored, printed in data/sets/M19.json, and tested on the real apply_action.
• Tests cover: blocking two attackers, damage assigned among them, and the second block still being refused for a creature without the permission.
• If requirements land, a test asserts a declaration meeting fewer requirements than possible is rejected, and that a restriction beats a requirement.
• Generated rules text covers both.
• The exclusion narrows or drops; make compat regenerated and committed.
• docs/card-schema.md and docs/protocol.md (prompt text) updated; make verify green.

[ninthworld]

Status check — this one may be closeable, but it names a mechanic that is still unbuilt, so I am flagging rather than closing.

Both cards are authored and both halves of the block work landed, before this tranche: a blocker blocking more than one attacker in #764 (Ghastbark Twins), and block requirements with the CR 509.1c maximisation search in #770 (Declare Dominance). Every acceptance criterion in this issue is met.

Attack requirements are not built, and are now named by their own exclusions.json entry: nothing can force a creature into the attacker declaration (CR 508.1d). No M19 card needs them.

So the question is whether an issue titled "attack and block requirements" closes when only the block half exists and the attack half is honestly excluded. Your call — closing it and letting attack requirements arrive with the card that needs them would match how the rest of this family has been handled.

[ninthworld]

Verified against main at f4c1779 — both parts landed and the issue was never closed.

• Blocking an additional creature — Ghastbark Twins authored; crates/sage-engine/tests/blocking_an_additional_creature.rs, 9 tests, covering a blocker blocking two attackers, damage assigned among them, and a second block still refused for a creature without the permission.
• Block requirements — Declare Dominance authored; crates/sage-engine/tests/block_requirements.rs, 8 tests. CR 509.1c maximisation over the whole declaration, with restrictions still winning over requirements.

34 tests green across the four files.

The exclusion narrowed to "Attack requirements" alone, which is the correct residue: nothing can force a creature into the attacker declaration (CR 508.1d), and the one block requirement modeled is every creature able to block this creature does so — never that one particular creature blocks. Both are separate from what this issue scoped.