Adv 2.0 D&D Phase 10 SUB2-AT: Arcane Trickster spellcasting

Slots into the Phase 9 spell pipeline so an L5+ Rogue who picks Arcane
Trickster becomes a third-caster on the Mage list, INT-based.

  - isSpellcaster(c) gate: full casters by class plus AT Rogue at L5+.
    !cast / !spells now consult this; the cast class-gate also accepts
    Mage-tagged spells when the player is an AT Rogue.
  - subclassSpellSlots() / slotsForCharacter() / setSpellSlotsForCharacter()
    layer subclass slots on top of the class baseline. AT pool: 3 L1 at
    L5–6, 4 L1 + 2 L2 at L7+, +2 L3 at L13+ (third-caster table).
  - spellcastingMod returns INT for AT Rogue.
  - L7 Magical Ambush proxy: spellSaveDC bumps +2 once Rogue+AT hits L7,
    standing in for "targets have disadvantage when casting from hidden"
    since the one-shot combat layer doesn't model hidden state.
  - ensureSpellsForCharacter bootstraps a Mage-list starter spellbook
    (minor_illusion, shocking_grasp, magic_missile, shield, sleep at L5;
    mirror_image + misty_step at L7+; hypnotic_pattern at L13+) and
    refreshes the slot pool on level/subclass change.
  - applySubclassChoice runs ensureSpellsForCharacter for AT so the
    spellbook is provisioned at selection time (idempotent).
  - !cast clamps upcasts to highestAvailableSlotForChar when AT, since
    third-caster's slot ceiling lags behind the spell list.

Tests added: AT spellcaster gate (L5 cutoff, non-AT Rogue rejected), INT
mod, subclass slot pool by tier, L7 ambush DC bump, ensureSpells default
list (all on Mage list, slots correct), end-to-end !cast queues
magic_missile for AT Rogue, !cast rejects plain Thief Rogue.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/plugin/dnd_cast.go

@@ -71,9 +71,9 @@ func (p *AdventurePlugin) handleDnDCastCmd(ctx MessageContext, args string) erro
 	if err != nil || c == nil {
 		return p.SendDM(ctx.Sender, "Couldn't load your Adv 2.0 sheet.")
 	}
-	if !classIsCaster(c.Class) {
+	if !isSpellcaster(c) {
 		return p.SendDM(ctx.Sender, fmt.Sprintf(
-			"%s isn't a caster class. `!cast` is for Mage, Cleric, and Ranger.",
+			"%s isn't a caster class. `!cast` is for Mage, Cleric, Ranger, and Arcane Trickster Rogues (L5+).",
 			titleClass(c.Class)))
 	}
 
@@ -119,10 +119,15 @@ func (p *AdventurePlugin) handleDnDCastCmd(ctx MessageContext, args string) erro
 			"Unknown spell %q. Run `!spells` to list what you know.", strings.Join(spellTokens, " ")))
 	}
 
-	// Class gate.
+	// Class gate. Arcane Trickster Rogues use the Mage spell list (Phase 10
+	// SUB2-AT), so accept Mage-tagged spells when the player has that subclass.
+	effectiveClass := c.Class
+	if c.Class == ClassRogue && c.Subclass == SubclassArcaneTrickster {
+		effectiveClass = ClassMage
+	}
 	classOK := false
 	for _, cl := range spell.Classes {
-		if cl == c.Class {
+		if cl == effectiveClass {
 			classOK = true
 			break
 		}
@@ -131,6 +136,14 @@ func (p *AdventurePlugin) handleDnDCastCmd(ctx MessageContext, args string) erro
 		return p.SendDM(ctx.Sender, fmt.Sprintf(
 			"%s is not on the %s spell list.", spell.Name, titleClass(c.Class)))
 	}
+	// AT slot ceiling — capped at L1 until L7 (then L2), tracks third-caster
+	// progression. Refuse upcasting beyond what the subclass actually has.
+	if c.Class == ClassRogue && c.Subclass == SubclassArcaneTrickster {
+		if max := highestAvailableSlotForChar(c); spell.Level > max {
+			return p.SendDM(ctx.Sender, fmt.Sprintf(
+				"Arcane Trickster L%d only has up to L%d slots.", c.Level, max))
+		}
+	}
 
 	// Reaction spells deferred.
 	if spell.Effect == EffectReaction {
@@ -368,7 +381,7 @@ func (p *AdventurePlugin) handleDnDSpellsCmd(ctx MessageContext, args string) er
 	if err != nil || c == nil {
 		return p.SendDM(ctx.Sender, "Couldn't load your Adv 2.0 sheet.")
 	}
-	if !classIsCaster(c.Class) {
+	if !isSpellcaster(c) {
 		return p.SendDM(ctx.Sender, fmt.Sprintf(
 			"%s isn't a caster class.", titleClass(c.Class)))
 	}