V2 Phase 3: combat rules automation (conditions, damage types, reminders)

Completes the deployable Phase 1-3 chunk: the combat tracker now runs the math.

- condition-effects engine: declarative CONDITION_EFFECTS_5E/PF2E table +
  deriveState(system, speed, conditions) → effective speed / advantage state /
  incapacitation / pf2e status penalty. Tracker shows effect badges
  ("Speed 0", "Disadv. attacks", "−2 status"); pure + 10 unit tests.
- damage types: applyDamage(c, amount, type?) consults snapshotted monster
  DamageDefenses (immune→0, resist→halve, vulnerable→double), back-compatible.
  Monster add snapshots damageDefenses; tracker shows a type picker only when a
  creature has typed defenses, and the log notes resisted/vulnerable amounts.
- concentration + death saves are surfaced as REMINDERS, never auto-rolled:
  damaging a concentrating PC logs "roll a DC N Con save…"; a downed PC's turn
  logs "make a death saving throw." The player rolls and resolves via the Drop
  button / death-save pips they already own. (Per user: no auto dice rolls —
  it removes the player from their character.)

Build + 271 unit tests green. Verified live: grapple → Speed 0 badge;
fire-immune creature shows the immune-labelled damage picker.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lib/mechanics/creatureState.ts

@@ -0,0 +1,88 @@
+import type { SystemId } from '@/lib/rules';
+import type { Condition } from '@/lib/schemas/common';
+import { conditionEffects } from './conditionEffects';
+import type { AbilityKey, AdvState } from './types';
+
+export interface MechanicalState {
+  /** effective speed after conditions (0 if any condition zeroes it) */
+  speed: number;
+  /** advantage state on THIS creature's attack rolls (5e) */
+  attackModifier: AdvState;
+  /** advantage state on attacks made AGAINST this creature (5e) */
+  attacksAgainstModifier: AdvState;
+  /** cannot take actions/reactions */
+  incapacitated: boolean;
+  /** per-ability save penalties (auto-fail dominates disadvantage) */
+  saveModifiers: Partial<Record<AbilityKey, 'disadvantage' | 'auto-fail'>>;
+  /** disadvantage on ability checks (5e) */
+  abilityCheckDisadvantage: boolean;
+  /** pf2e: aggregated numeric status penalty (worst single value, penalties don't stack) */
+  statusPenalty: number;
+  /** short human-readable badges for the UI, e.g. "Speed 0", "Disadv. attacks" */
+  badges: string[];
+}
+
+/** Combine two advantage states by the 5e rule: opposing sources cancel to normal. */
+function combineAdv(a: AdvState, b: AdvState): AdvState {
+  if (a === b) return a;
+  if (a === 'normal') return b;
+  if (b === 'normal') return a;
+  return 'normal'; // one advantage + one disadvantage cancel
+}
+
+/**
+ * Fold a creature's conditions into a single mechanical read-model. Pure and
+ * synchronous — combat, the dice roller, and the player view all call this
+ * (the player view ships the same table and derives effects locally from the
+ * broadcast condition names, so no derived state crosses the wire).
+ *
+ * Unknown / homebrew condition names contribute nothing (they remain visible
+ * labels), exactly as before.
+ */
+export function deriveState(system: SystemId, baseSpeed: number, conditions: Condition[]): MechanicalState {
+  const table = conditionEffects(system);
+  let speedZero = false;
+  let attackModifier: AdvState = 'normal';
+  let attacksAgainstModifier: AdvState = 'normal';
+  let incapacitated = false;
+  let abilityCheckDisadvantage = false;
+  let statusPenalty = 0;
+  const saveModifiers: Partial<Record<AbilityKey, 'disadvantage' | 'auto-fail'>> = {};
+
+  for (const cond of conditions) {
+    const eff = table[cond.name.trim().toLowerCase()];
+    if (!eff) continue;
+    if (eff.speedZero) speedZero = true;
+    if (eff.incapacitated) incapacitated = true;
+    if (eff.abilityChecks) abilityCheckDisadvantage = true;
+    if (eff.attackRolls) attackModifier = combineAdv(attackModifier, eff.attackRolls);
+    if (eff.attacksAgainst) attacksAgainstModifier = combineAdv(attacksAgainstModifier, eff.attacksAgainst);
+    if (eff.saves) {
+      for (const [ability, level] of Object.entries(eff.saves) as [AbilityKey, 'disadvantage' | 'auto-fail'][]) {
+        // auto-fail dominates disadvantage
+        if (level === 'auto-fail' || saveModifiers[ability] === undefined) saveModifiers[ability] = level;
+      }
+    }
+    // pf2e: status penalties don't stack — take the worst single value.
+    if (eff.statusPenalty && cond.value !== undefined) statusPenalty = Math.max(statusPenalty, cond.value);
+  }
+
+  const badges: string[] = [];
+  if (speedZero) badges.push('Speed 0');
+  if (incapacitated) badges.push('Incapacitated');
+  if (attackModifier === 'disadvantage') badges.push('Disadv. attacks');
+  if (attackModifier === 'advantage') badges.push('Adv. attacks');
+  if (attacksAgainstModifier === 'advantage') badges.push('Attacked w/ adv.');
+  if (statusPenalty > 0) badges.push(`−${statusPenalty} status`);
+
+  return {
+    speed: speedZero ? 0 : baseSpeed,
+    attackModifier,
+    attacksAgainstModifier,
+    incapacitated,
+    saveModifiers,
+    abilityCheckDisadvantage,
+    statusPenalty,
+    badges,
+  };
+}