feat: add driving flag to relationship query responses

Add computed driving flag to all relationship queries (list_relationships,
get_predecessors, get_successors). A relationship is marked as driving when
the predecessor's early end date plus lag determines the successor's early
start date.

Changes:
- Add early_start_date and early_end_date columns to activities schema
- Parse early dates from TASK table in XER files
- Implement is_driving_relationship() helper with 24hr tolerance for
  calendar gaps
- Update all relationship queries to compute and return driving flag
- Add contract and unit tests for driving flag functionality
- Update spec, contracts, and documentation

src/xer_mcp/parser/table_handlers/task.py

@@ -36,6 +36,8 @@ class TaskHandler(TableHandler):
             "status_code": data.get("status_code") or None,
             "target_start_date": convert_date(data.get("target_start_date")),
             "target_end_date": convert_date(data.get("target_end_date")),
+            "early_start_date": convert_date(data.get("early_start_date")),
+            "early_end_date": convert_date(data.get("early_end_date")),
             "act_start_date": convert_date(data.get("act_start_date")),
             "act_end_date": convert_date(data.get("act_end_date")),
             "total_float_hr_cnt": total_float,