feat: add Invocation System spec for Invoker attunement

- New spec at docs/specs/attunements/invoker/systems/invocation-system-spec.md
- Invocation Charge meter (0-100) fills passively, drains while invoking
- Channeled guardian auto-casts spells from their elements at reduced cost
- Spell selection picks best affordable spell, steps down as mana depletes
- No guardian swap mid-invocation; ends when charge depleted or room cleared
- Pact Affinity extended with diminishing-returns cast speed bonus (max 50%)
- New guardian-invocation discipline with 4 perks (efficiency, speed, sustain, mastery)

docs/specs/attunements/invoker/systems/invocation-system-spec.md

@@ -177,25 +177,27 @@ spell in `SPELLS_DEF` where `spell.elem` is `'metal'`, `'fire'`, or `'earth'`.
 
 ### 5.2 Spell Selection Algorithm
 
-On activation and after each cast, the invocation spell is re-evaluated:
+On activation and after each cast, the invocation spell is re-evaluated.
+The player is channeling the **guardian's** power — the invocation spellbook is
+all spells from the guardian's elements, regardless of what the player has
+personally learned:
 
 ```
-1. Gather all spells from the guardian's elements
-2. Filter to spells the player has learned (spells[spellId].learned === true)
-3. Filter to spells the player can afford at the effective cost multiplier:
+1. Gather all spells from the invoked guardian's elements (the full guardian spellbook)
+2. Filter to spells the player can afford at the effective cost multiplier:
    - For raw cost: rawMana >= cost.amount × effectiveCostMultiplier
    - For element cost: elements[element].current >= cost.amount × effectiveCostMultiplier
-4. From remaining spells, pick the one with the highest base damage (spell.dmg)
-5. If tied, pick the highest tier
-6. If still tied, pick the lowest cost (most efficient)
+3. From remaining spells, pick the one with the highest base damage (spell.dmg)
+4. If tied, pick the highest tier
+5. If still tied, pick the lowest cost (most efficient)
 ```
 
-### 5.3 Fallback and Auto-End
+### 5.3 Auto-End When Unaffordable
 
-If no spells are affordable at the effective cost multiplier from the current
-guardian:
-1. Try the next guardian in the scoring order (§3.2)
-2. If no guardian has affordable spells, invocation ends (§3.3, condition 4)
+The invoked guardian is fixed for the entire invocation — the player cannot swap
+to a different guardian mid-invocation. If the player cannot afford **any** spell
+from the **invoked guardian's** elements at the effective cost multiplier,
+invocation ends immediately (§3.3, condition 4).
 
 ### 5.4 Invocation Spell Cast Slot
 
@@ -207,7 +209,7 @@ In `processCombatTick`, the invocation spell is processed similarly to equipment
 spells:
 - It has its own `castProgress` accumulator
 - It uses the same `HOURS_PER_TICK × spellCastSpeed × effectiveAttackSpeed` progress formula
-- On cast completion, it deducts mana at the effective cost multiplier
+- On cast **start**, it deducts mana at the effective cost multiplier (same pattern as the active spell)
 - Damage is calculated using the existing `calcDamage()` with the invocation spell's element
 
 The invocation spell does **not** interfere with the player's active spell. Both
@@ -439,7 +441,6 @@ export function getGuardianSpellbook(
 // Select the best affordable spell from a spellbook
 export function selectInvocationSpell(
   spellbook: SpellDef[],
-  playerSpells: Record<string, SpellState>,
   rawMana: number,
   elements: Record<string, { current: number; max: number; unlocked: boolean }>,
   costMultiplier: number,
@@ -476,11 +477,10 @@ spell block, add an **invocation block**:
    b. If charge <= 0: end invocation (but let current cast finish)
    c. Accumulate cast progress for invocation spell
    d. On cast completion:
-      - Deduct mana at effectiveCostMultiplier
       - Calculate damage using calcDamage() with invocation spell element
       - Apply damage via applyDamageToRoom()
       - Re-evaluate spell selection (§5.2)
-      - If no affordable spell from any guardian: end invocation
+      - If no affordable spell from the invoked guardian: end invocation
    e. If room cleared: end invocation
 
 2. If activeInvocation === null AND invocationCharge >= 100
@@ -563,9 +563,9 @@ gameStore.tick()
 | AC-1 | `invocationCharge` fills passively at `baseFillRate × pactCountMultiplier × disciplineMultiplier` per tick while in `climb` action and not invoking. |
 | AC-2 | Invocation auto-activates when charge ≥ 100, `activeInvocation === null`, and room has enemies. |
 | AC-3 | The channeled guardian is selected by `bestElementalBonus × tierMultiplier` scoring. |
-| AC-4 | The invocation spell is the highest-damage learned spell the player can afford at the effective cost multiplier. |
+| AC-4 | The invocation spell is the highest-damage spell from the guardian's elements that the player can afford at the effective cost multiplier (not limited to spells the player has learned). |
 | AC-5 | Invocation spell steps down to the next affordable spell when the current one becomes unaffordable. |
-| AC-6 | If no guardian has affordable spells, invocation ends. |
+| AC-6 | If no spells are affordable from the invoked guardian's elements, invocation ends. |
 | AC-7 | Invocation ends when charge reaches 0, room is cleared, or player leaves `climb`. |
 | AC-8 | A cast already in progress completes even if charge hits 0 during the cast. |
 | AC-9 | After invocation ends, charge must fully recharge to 100 before invocation can reactivate. |