docs: Add README.md, update AGENTS.md, audit report, and massive refactoring

Documentation:
- Add comprehensive README.md with project overview
- Update AGENTS.md with new file structure and slice pattern
- Add AUDIT_REPORT.md documenting unimplemented effects

Refactoring (page.tsx: 1695 → 434 lines, 74% reduction):
- Extract SkillsTab.tsx component
- Extract StatsTab.tsx component
- Extract UpgradeDialog.tsx component
- Move getDamageBreakdown and getTotalDPS to computed-stats.ts
- Move ELEMENT_ICON_NAMES to constants.ts

All lint checks pass, functionality preserved.

src/lib/game/computed-stats.ts

@@ -11,6 +11,8 @@ import {
   MAX_DAY,
   INCURSION_START_DAY,
   ELEMENT_OPPOSITES,
+  ELEMENTS,
+  TICK_MS,
 } from './constants';
 import type { ComputedEffects } from './upgrade-effects';
 import { getUnifiedEffects, type UnifiedEffects } from './effects';
@@ -395,3 +397,95 @@ export function deductSpellCost(
   
   return { rawMana, elements: newElements };
 }
+
+// ─── Damage Breakdown Helper ───────────────────────────────────────────────────
+
+export interface DamageBreakdown {
+  base: number;
+  combatTrainBonus: number;
+  arcaneFuryMult: number;
+  elemMasteryMult: number;
+  guardianBaneMult: number;
+  pactMult: number;
+  precisionChance: number;
+  elemBonus: number;
+  elemBonusText: string;
+  total: number;
+}
+
+export function getDamageBreakdown(
+  state: Pick<GameState, 'skills' | 'signedPacts'>,
+  activeSpellId: string,
+  floorElem: string,
+  isGuardianFloor: boolean
+): DamageBreakdown | null {
+  const spell = SPELLS_DEF[activeSpellId];
+  if (!spell) return null;
+  
+  const baseDmg = spell.dmg;
+  const combatTrainBonus = (state.skills.combatTrain || 0) * 5;
+  const arcaneFuryMult = 1 + (state.skills.arcaneFury || 0) * 0.1;
+  const elemMasteryMult = 1 + (state.skills.elementalMastery || 0) * 0.15;
+  const guardianBaneMult = isGuardianFloor ? (1 + (state.skills.guardianBane || 0) * 0.2) : 1;
+  const pactMult = state.signedPacts.reduce((m, f) => m * (GUARDIANS[f]?.pact || 1), 1);
+  const precisionChance = (state.skills.precision || 0) * 0.05;
+  
+  // Elemental bonus
+  let elemBonus = 1.0;
+  let elemBonusText = '';
+  if (spell.elem !== 'raw' && floorElem) {
+    if (spell.elem === floorElem) {
+      elemBonus = 1.25;
+      elemBonusText = '+25% same element';
+    } else if (ELEMENTS[spell.elem] && ELEMENT_OPPOSITES[floorElem] === spell.elem) {
+      elemBonus = 1.5;
+      elemBonusText = '+50% super effective';
+    }
+  }
+  
+  return {
+    base: baseDmg,
+    combatTrainBonus,
+    arcaneFuryMult,
+    elemMasteryMult,
+    guardianBaneMult,
+    pactMult,
+    precisionChance,
+    elemBonus,
+    elemBonusText,
+    total: calcDamage(state, activeSpellId, floorElem)
+  };
+}
+
+// ─── Total DPS Calculation ─────────────────────────────────────────────────────
+
+export function getTotalDPS(
+  state: Pick<GameState, 'skills' | 'signedPacts' | 'equippedInstances' | 'equipmentInstances'>,
+  upgradeEffects: { attackSpeedMultiplier: number },
+  floorElem: string
+): number {
+  const quickCastBonus = 1 + (state.skills.quickCast || 0) * 0.05;
+  const attackSpeedMult = upgradeEffects.attackSpeedMultiplier;
+  const castsPerSecondMult = HOURS_PER_TICK / (TICK_MS / 1000);
+  
+  const activeEquipmentSpells = getActiveEquipmentSpells(
+    state.equippedInstances,
+    state.equipmentInstances
+  );
+  
+  let totalDPS = 0;
+  
+  for (const { spellId } of activeEquipmentSpells) {
+    const spell = SPELLS_DEF[spellId];
+    if (!spell) continue;
+    
+    const spellCastSpeed = spell.castSpeed || 1;
+    const totalCastSpeed = spellCastSpeed * quickCastBonus * attackSpeedMult;
+    const damagePerCast = calcDamage(state, spellId, floorElem);
+    const castsPerSecond = totalCastSpeed * castsPerSecondMult;
+    
+    totalDPS += damagePerCast * castsPerSecond;
+  }
+  
+  return totalDPS;
+}