i18n: campaign level titles/scenarios/debriefs are EN-only data, not translatable

[pshenok]

Found while shipping the Hindi locale (#237): the campaign's most-read text — level title, scenario, learn, debriefTip, plus objective labels — lives as English strings in src/campaign/levels.js data (the schema even documents them as "UI strings (EN)"). They render in English for every locale, including uk and the new hi.

For any locale, this is now the single biggest localization gap: the campaign is the education spine, and its narrative is the part players actually read.

Scope

25 levels × (title + scenario + learn + debriefTip + ~4 objective labels) ≈ ~200 keys × 11 locales ≈ 2200 strings, plus the mechanism:

• Move level strings to i18n keys (level_<id>_title, level_<id>_scenario, …, obj_<levelId>_<objId>), keep levels.js holding keys not prose
• The level-invariant tests should assert every level's keys exist in en (the raw-key protection)
• Translate per-locale following each locale's established register (hi = Hinglish rules from feat: Hindi locale — 705 keys in the Hinglish register (हिन्दी) #237's glossary; uk = ти-tone glossary from Add Ukrainian locale (uk) #230)
• The beatability harness and objective check functions are untouched — this is presentation-only

Worth doing in two PRs: (1) the mechanism + en extraction (zero visible change), (2) the translations.

[pshenok]

Done in two parts as planned: #239 (mechanism + EN extraction, self-verifying codegen, three new CI gates) and #240 (translations for all 10 locales via per-locale translator → native-reviewer pipelines, 126 review findings fixed). The campaign narrative now renders in the player's language everywhere — briefing, tooltip, HUD objectives, debrief, and the fail reasons.