[Fix] message: bound per-part newline metadata

[Fix] message: bound per-part newline metadata

Newline normalization recorded every newline as a pointer array entry
plus a mempool exception object and a GList node, then sorted the
whole exceptions list: a newline-dense body turned a few MiB of input
into hundreds of MiB of heap and millions of allocations. Cap the
recorded newline positions at 100k per part (text normalization and
line counters are unaffected), set a part flag on truncation and
expose it as newlines_truncated in textpart:get_stats().