SymbolTableNode::parse used raw offset+8 arithmetic that can overflow
on a crafted v1-group B-tree leaf with a near-u64::MAX SNOD child
pointer (group_v1.rs passes such offsets through unchecked). Switch to
checked_add, matching read_offset in the same file. Also harden the
entries_start + num_symbols*entry_size computation with checked_add
for consistency, even though num_symbols being u16 already bounds
that multiply. Add regression tests.
INT-02
ClawHDF5 Coding Agent