Docs: Record the milestones and their dates in PROJECT.md #14

The maintainer asked for a completion date per milestone. Three GitHub milestones exist (M1 2026-10-16, M2 2026-11-13, M3 2026-11-27) with issues #7–#13. Done when PROJECT.md carries the same table (name, date, scope, completion criterion, issues) with the reasoning behind each date, and the test counts are unchanged (documentation only).