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

docs/project-structure.txt

@@ -20,7 +20,8 @@ Mana-Loop/
 │   ├── tasks/
 │   │   ├── TASK-001-playwright-setup.md
 │   │   ├── TASK-005-globals-css-tokens.md
-│   │   └── TASK-006-left-panel-redesign.md
+│   │   ├── TASK-006-left-panel-redesign.md
+│   │   └── TASK-007-skill-system-v2.md
 │   ├── GAME_BRIEFING.md
 │   ├── active-task-log.md
 │   ├── circular-deps.txt
@@ -274,6 +275,19 @@ Mana-Loop/
 │       │   │   ├── index.ts
 │       │   │   ├── prestige.ts
 │       │   │   ├── rooms.ts
+│       │   │   ├── skills-combat.ts
+│       │   │   ├── skills-core.ts
+│       │   │   ├── skills-crafting.ts
+│       │   │   ├── skills-element-caps.ts
+│       │   │   ├── skills-enchant.ts
+│       │   │   ├── skills-golemancy.ts
+│       │   │   ├── skills-hybrid.ts
+│       │   │   ├── skills-invocation.ts
+│       │   │   ├── skills-research.ts
+│       │   │   ├── skills-v2-defs.ts
+│       │   │   ├── skills-v2-registry.ts
+│       │   │   ├── skills-v2-types.ts
+│       │   │   ├── skills-v2.ts
 │       │   │   ├── skills.ts
 │       │   │   └── spells.ts
 │       │   ├── crafting-actions/
@@ -434,6 +448,7 @@ Mana-Loop/
 │       │   │   │   │   ├── spell-definitions.test.ts
 │       │   │   │   │   └── study-speed.test.ts
 │       │   │   │   ├── ui-store-tests/
+│       │   │   │   ├── computed-stats.test.ts
 │       │   │   │   ├── equipment.test.ts
 │       │   │   │   ├── mana-conversion-fix.test.ts
 │       │   │   │   ├── mana.test.ts