Use unique point IDs across all phases instead of overwriting the same
id repeatedly. The gridstore payload storage size estimate is
bitmask-based: overwrites keep old blocks allocated until a periodic
flush reclaims them, so the test was racing the 5s flush worker. With
unique ids every block stays live and the post-flush size still
reflects all inserted points.
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>