fix(get-report-data): use real fixable counts from core (A1)

[luojiyin1987]

Summary

• get-report-data.ts hardcoded per-file fixableErrorCount / fixableWarningCount to 0, overwriting @lint-md/core's real values, so the "potentially fixable with --fix" summary was always empty.
• Forward the real counts from lintMarkdown's result through lint-worker into BatchLintItem (new optional fixableErrorCount? / fixableWarningCount? fields), and use them in getReportData (res.fixableErrorCount ?? 0). The existing aggregation/summary code was already correct — it was just fed zeros.
• Also populate the counts for the --stdin report path.

Verification

• npm run build (tsc cjs + esm) passes.
• npm test: 11 suites / 106 tests pass (6 new get-report-data tests covering error/warning counting, multi-file aggregation, and fixable summary presence/absence).