Implement the 005-set-initiative feature that adds initiative values to combatants with automatic descending sort and active turn preservation

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

specs/005-set-initiative/quickstart.md

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+# Quickstart: Set Initiative
+
+## What This Feature Does
+
+Adds an optional initiative value to combatants. When set, the encounter automatically sorts combatants from highest to lowest initiative. Combatants without initiative appear at the end. The active turn is preserved through reorders.
+
+## Key Files to Modify
+
+1. **`packages/domain/src/types.ts`** — Add `initiative?: number` to `Combatant`
+2. **`packages/domain/src/events.ts`** — Add `InitiativeSet` event to the union
+3. **`packages/domain/src/set-initiative.ts`** — New domain function (pure, no I/O)
+4. **`packages/domain/src/index.ts`** — Export new function and types
+5. **`packages/application/src/set-initiative-use-case.ts`** — New use case
+6. **`packages/application/src/index.ts`** — Export use case
+7. **`apps/web/src/hooks/use-encounter.ts`** — Add `setInitiative` callback
+8. **`apps/web/src/App.tsx`** — Add initiative input next to each combatant
+
+## Implementation Order
+
+1. Domain types + event (foundation)
+2. Domain function + tests (core logic)
+3. Application use case (orchestration)
+4. Web adapter hook + UI (user-facing)
+
+## How to Verify
+
+```bash
+pnpm check    # Must pass: format + lint + typecheck + test
+```
+
+## Patterns to Follow
+
+- Domain functions return `{ encounter, events } | DomainError` — never throw
+- Use `readonly` everywhere, create new objects via spread
+- Tests live in `packages/domain/src/__tests__/`
+- Use cases follow get → call → check error → save → return events