fix(dl): avoid export JSON parse race

[expary]

Summary

• read Telegram export chat metadata with a synchronous JSON decoder
• parse message entries from a separate file handle to avoid shared offset races
• add regression coverage for repeated parsing of a 5000-message export

Tests

• go test ./pkg/tmessage -count=50
• go test $(go list ./... | grep -v '^github.com/iyear/tdl/test$') ./core/... ./extension/...

Closes #1242