feat(config): warn on unknown log levels and accept upper-case LOGIXIA_LEVEL #29

Context

A small piece of #7 (config validation).

resolveInitialLevel (src/core/logitron-logger.ts line 131) takes process.env.LOGIXIA_LEVEL or levelOptions.level as-is. The numeric lookup in _buildPerfCaches (around line 336) then does this._levelValues.get(effectiveLevel) ?? 2. So:

• LOGIXIA_LEVEL=DEBUG (upper case) silently becomes info, because the level map keys are lower case
• a typo like levelOptions: { level: 'warning' } silently becomes info
• setLevel('nope') (line 510) keeps the old threshold but stores 'nope', so getLevel() returns a level that isn't in effect

What to do

• Normalise the resolved level to lower case in resolveInitialLevel and in setLevel
• When the level isn't in _levelValues (built-ins plus custom levels), call internalWarn once per logger with the bad value and the list of valid levels, then fall back to info as today
• In setLevel, refuse unknown levels: warn and keep the current level, so getLevel() stays truthful
• Tests in src/core/__tests__/level-validation.test.ts:
  • LOGIXIA_LEVEL=DEBUG enables debug
  • unknown level warns once and logs at info
  • a custom level such as kafka is accepted
  • setLevel('nope') warns and getLevel() is unchanged

Acceptance criteria

• npx jest src/core/__tests__/level-validation.test.ts passes, and the full npx jest still passes
• npm run typecheck && npm run lint pass

Hints

• internalWarn is in src/utils/internal-log.ts. Tests can silence or capture it, see src/utils/__tests__/internal-log.test.ts.
• src/core/__tests__/custom-levels.test.ts shows how custom levels are configured.