fix(campaign): register c50 with the fields the app actually reads

Opening the c50 edition threw `campaign.legacyStorageKeys is not iterable`
and left the board blank — the header still drew, so the level set looked
loaded. Every other edition carries the array; the hand-written c50 entry
did not, and 197 passing tests had nothing to say about it, because nothing
tested the registration itself.

The test now walks every edition and asserts the shape app.js relies on,
including that the chapter list can actually hold the levels. It fails on the
entry as it shipped.

The task board was reading levels/prototypes/d5.json while the campaign sat in
levels/campaign-50.json, so it showed "候选关卡 18 / 50" with fifty levels
built. It reads the shipping set when there is one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/campaign-versions.js

@@ -36,6 +36,7 @@ export const CAMPAIGN_VERSIONS = [
   {
     id:C50_CAMPAIGN.id, name:C50_CAMPAIGN.name, description:C50_CAMPAIGN.description,
     chapters:C50_CAMPAIGN.chapters, chapterSize:C50_CAMPAIGN.chapterSize, levels:C50_LEVELS,
+    legacyStorageKeys:[],
   },
 ];
 export const DEFAULT_CAMPAIGN_VERSION = 'v0.5';