Build MVP: campaigns, characters, combat, dice, compendium

- Rules abstraction (5e + pf2e) behind one interface, fully tested
- Pure combat engine: turn-order safe across add/remove/reorder/init-change
- Dexie storage with real cascade deletes + Zod validation on write
- Seedable dice engine with notation parser
- Lazy SRD compendium (code-split), bestiary -> combat
- Strict TS, 54 unit tests, Playwright e2e smoke (all green)

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

src/lib/format.ts

@@ -0,0 +1,4 @@
+/** Format a modifier with an explicit sign, e.g. 3 -> "+3", -1 -> "-1". */
+export function formatModifier(n: number): string {
+  return n >= 0 ? `+${n}` : String(n);
+}