Reject mismatched point counts in legacy KD-tree streams

[carrerasdarren-cell]

Summary

• reuse the bounded point decoder for legacy (pre-2.3) integer KD-tree streams
• reject disagreement between the outer attribute point count and the nested KD-tree count
• add a regression test using the public OSS-Fuzz reproducer

Impact

The reproducer declares 168 points in the outer legacy attribute stream but 3,892,261,426 points in the nested KD-tree stream. The legacy path called the unbounded decoder overload, while its output iterator silently ignores writes beyond the allocated 168 entries. The decoder therefore continued processing the attacker-controlled nested count.

On an ASan build from 4723893, the input was accepted after 10.04 seconds. With this change, it is rejected before entropy decoding in 0.04 seconds.

Testing

• PointCloudKdTreeEncodingTest.*: 10/10 tests pass under ASan
• exact OSS-Fuzz reproducer is rejected
• 14 valid bitstream-2.2 KD-tree files generated by Draco 1.2.5 still decode, covering compression settings 1-10 and position-plus-color attributes
• clang-format --dry-run --Werror
• git diff --check

Fixes #1207