engine: an observed activation cannot say "you"

[ninthworld]

Goal

A card that says whenever you activate an ability watches only its controller's
activations, so Sarkhan's Whelp stops firing on an opponent's Sarkhan.

Context and evidence

Parent: #819.

ActivatorScope in crates/sage-engine/src/ability/trigger.rs has exactly two members:

pub enum ActivatorScope {
    /// Any player's activation, the watcher's own controller included.
    #[default]
    Any,
    /// Only an opponent of the watcher's controller …
    Opponents,
}

Sarkhan's Whelp prints Whenever you activate an ability of a Sarkhan planeswalker, this
creature deals 1 damage to any target. There is no scope for "you", so
sarkhan_s_whelp.json is authored "activator": "any" — and an opponent activating their own
Sarkhan fires your Whelp.

This is the eighteenth wrong card in the audit, and the only one that is a mechanic
finding rather than an authoring one: the vocabulary offers no correct answer, so no amount of
care in the JSON would have got it right.

exclusions.json does not name the gap. Its nearest entry — "Abilities that trigger on a
loyalty ability specifically" — asserts in its blocker text that the activation filter
covers "who activated it", which is what makes the missing scope invisible. Per
crates/sage-engine/AGENTS.md, that file "claims to name every mechanic considered and left
out of scope, so a gap that is neither built nor named makes it false."

Building it is the right fix rather than naming it, and that is also the direction #791 is
taking the exclusion registry as a whole. It is a one-variant change: ActivatorScope is a
small Copy enum consumed in one predicate.

Related: #776 item 7 notes the adjacent gap — nothing filters an observed activation by
whether it was a loyalty ability. Distinct question, same enum, and worth resolving in the
same read even if it is left to the card that needs it.

Scope

• ActivatorScope gains the controller-only scope, documented alongside Any and Opponents
  and scoped from the same "you" every other trigger scope is written from.
• The predicate that reads it follows; it is wildcard-free, so the compiler names the site.
• sarkhan_s_whelp.json is corrected.
• The server's rules-text formatter renders the new scope.
• exclusions.json: the loyalty entry's blocker text no longer implies the activator filter is
  complete. Coordinate with Remove exclusions.json and prohibit card/mechanic denylists #791 — if the registry is being removed, this is a deletion rather
  than an edit.

Acceptance criteria

• An opponent activating a Sarkhan planeswalker they control does not fire
  sarkhan_s_whelp.
• The controller activating a Sarkhan planeswalker they control does fire it, once per
  activation.
• A mana ability of a Sarkhan still fires nothing — the existing structural exclusion
  (a mana ability never reaches the stack, CR 605.3a) is unaffected.
• The rendered rules text for sarkhan_s_whelp says "you", not "a player".
• exclusions.json no longer asserts the activator filter is complete.
• make compat regenerated; make verify green.

Test and validation evidence

• An engine test with a Sarkhan on each side, activating both, asserting one fire.
• A rules-text test for the new scope's rendering.
• make compat drift check; make verify.

Dependencies and sequencing

Independent of every other child of #819 and small enough to land at any point. Coordinate
with #791 on the exclusions.json half only.

Estimated scope

Small — localized change: one enum variant, one predicate arm, one formatter arm, one JSON
file.

Risks and constraints

None material. The scope is Copy, closed, and read in one place.

Non-goals

Not the loyalty-ability filter of #776 item 7 — related, and fine to do here if it is free, but
it blocks no card. Not a general rework of the trigger vocabulary.

Documentation impact

docs/card-schema.md if the trigger vocabulary's scopes are enumerated there.
exclusions.json as above. No ADR.