chore: bump version to 0.15.1, add comprehensive scheduling tests

Version bump: 0.15.0 → 0.15.1 (PATCH for bug fix)

Added 14 new tests in complete_task_scheduling.rs covering:
- Regression tests for the fixed bug (sequential complete tasks)
- Three-task chains without actual dates
- Mixed scenarios (some with actual dates, some without)
- Parallel complete tasks
- Diamond dependencies with all complete tasks
- actual_finish only (no actual_start)
- Mixed progress states (Complete → InProgress → NotStarted)
- Container with complete children
- Slack calculation verification
- Long chain stress test (20 tasks)

Total test count: 978 tests passing

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: 2 people

Cargo.lock

@@ -14,7 +14,7 @@ members = [
 ]
 
 [workspace.package]
-version = "0.15.0"
+version = "0.15.1"
 edition = "2021"
 rust-version = "1.75"
 license = "MIT OR Apache-2.0"