engine: naming a colour, a type, or a card

[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): Effects that ask a player to name a colour, type, or card — no player choice recorded on a permanent or spell.

The mid-resolution choice queue (ADR 0013) already asks players questions and already has an answer_color, which add_mana_any_color uses. What is missing is a choice that is recorded on the permanent and read for as long as it is on the battlefield.

M19 cards blocked (3)

#	Card	Choice, and when it is made
231	Diamond Mare	a colour, as it enters; later cast triggers read it
128	Alpine Moon	a land card name, as it enters (also needs loses-all-abilities and a static reaching opponents' permanents)
60	Metamorphic Alteration	a creature, as the Aura enters (also needs copy effects)

Scope

1. "As this enters" is not an enters-the-battlefield trigger: the choice is made as part of the permanent arriving (CR 614.12), before any ability could respond, and the permanent is never briefly on the battlefield without it. The engine already has that seam — enters_tapped, enters_with_counters, and planeswalker loyalty all ride it.
2. The answer must be state on the permanent, so it survives every read, is projected in GameView, and is visible to the abilities that consult it.
3. Naming a card is the one that must not leak the legal posture: store a functional identity, never prose, and never a name the catalog does not contain.
4. Author Diamond Mare — it is the smallest card that records a choice and reads it back, and its cast trigger filtered by colour is a small widening of you_cast_spell.

Acceptance criteria

• Diamond Mare authored, printed in data/sets/M19.json, and tested on the real apply_action.
• Tests cover: the choice is made on entry, is projected to both seats, and drives the trigger for the chosen colour and not for others.
• Generated rules text names the choice; the client renders the recorded answer with no inference.
• The exclusion narrows; make compat regenerated and committed.
• docs/card-schema.md and docs/decisions/0013-mid-resolution-player-choices.md updated; make verify green.

[ninthworld]

Progress in #775 (merged as 443c66f). Two of the three choices now land, and the exclusion is down to naming a type.

Built: a colour named as a permanent enters (Diamond Mare, #765), and now a card named as one enters (Alpine Moon). The card is stored as a CardId — a functional identity chosen from a server-computed candidate list derived from the catalog on every read and sorted so replays are identical. No string ever travels, apply_action re-checks membership, and a forged handle, a basic land, and a non-land are all refused. The legal posture this issue worried about is held by construction.

Still missing: naming a card or creature type, and any choice recorded on a spell rather than a permanent. Also note only a nonbasic land may currently be named — the class is not yet general.

Metamorphic Alteration's "name a creature" is a different mechanism (a permanent on the battlefield, not a card name) and shipped with #734.

[ninthworld]

Verified against main at f4c1779 — this landed and was never closed.

All three blocked cards are authored and tested on the real apply_action:

• Diamond Mare (colour recorded as it enters, read by the cast trigger) — crates/sage-engine/tests/m19_diamond_mare.rs, 9 tests
• Alpine Moon (a land card named as it enters, stored as a functional identity — never prose) — crates/sage-engine/tests/m19_alpine_moon.rs, 8 tests
• Metamorphic Alteration — crates/sage-engine/tests/copy_effects.rs

The choice is projected in GameView (view/cards/entry_choice_tests.rs) and named in generated rules text (rules_text/tests.rs), so the client renders the recorded answer with no inference.

The exclusion narrowed rather than dropped, as the issue anticipated: it now reads "Effects that ask a player to name a type" — a permanent records a colour and a card, but a card or creature type still has no recorded identity, and nothing on a spell records a choice at all. That residue belongs to the card that needs it, per the standing one-card-at-a-time policy.