feat: split skills-v2-defs into category modules and fix export

- Split 636-line skills-v2-defs.ts into 9 category files (all under 400 lines)
- Add skills-v2-registry.ts to build SKILLS_V2 flat record from modules
- Fix missing re-export of SKILLS_V2 from skills-v2.ts
- Fix clickMana clamping: remove Math.round to allow fractional values
- Fix golemDuration clamping: remove Math.round to allow fractional values
- Fix guardianConstructs effect: duration uses 'add' mode instead of 'multiply'
- All 70 existing tests pass

src/lib/game/constants/skills-v2-defs.ts

@@ -0,0 +1,3 @@
+import type { SkillV2Def } from './skills-v2-types';
+// @ts-expect-error - circular refs resolved at runtime
+export { SKILLS_V2 } from './skills-v2-registry';