feat: implement XER MCP Server with 9 schedule query tools

Implement complete MCP server for parsing Primavera P6 XER files and
exposing schedule data through MCP tools. All 4 user stories complete.

Tools implemented:
- load_xer: Parse XER files into SQLite database
- list_activities: Query activities with pagination and filtering
- get_activity: Get activity details by ID
- list_relationships: Query activity dependencies
- get_predecessors/get_successors: Query activity relationships
- get_project_summary: Project overview with counts
- list_milestones: Query milestone activities
- get_critical_path: Query driving path activities

Features:
- Tab-delimited XER format parsing with pluggable table handlers
- In-memory SQLite database for fast queries
- Pagination with 100-item default limit
- Multi-project file support with project selection
- ISO8601 date formatting
- NO_FILE_LOADED error handling for all query tools

Test coverage: 81 tests (contract, integration, unit)

src/xer_mcp/parser/table_handlers/__init__.py

@@ -0,0 +1,27 @@
+"""XER table handlers module."""
+
+from xer_mcp.parser.table_handlers.base import TableHandler
+from xer_mcp.parser.table_handlers.calendar import CalendarHandler
+from xer_mcp.parser.table_handlers.project import ProjectHandler
+from xer_mcp.parser.table_handlers.projwbs import ProjwbsHandler
+from xer_mcp.parser.table_handlers.task import TaskHandler
+from xer_mcp.parser.table_handlers.taskpred import TaskpredHandler
+
+# Registry mapping table names to handlers
+TABLE_HANDLERS: dict[str, type[TableHandler]] = {
+    "PROJECT": ProjectHandler,
+    "TASK": TaskHandler,
+    "TASKPRED": TaskpredHandler,
+    "PROJWBS": ProjwbsHandler,
+    "CALENDAR": CalendarHandler,
+}
+
+__all__ = [
+    "CalendarHandler",
+    "ProjectHandler",
+    "ProjwbsHandler",
+    "TABLE_HANDLERS",
+    "TableHandler",
+    "TaskHandler",
+    "TaskpredHandler",
+]