feat: add map step type in sql

.changeset/yummy-geckos-marry.md

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+---
+'@pgflow/core': patch
+---
+
+Add map step type infrastructure in SQL core
+
+## 🚨🚨🚨 CRITICAL MIGRATION WARNING 🚨🚨🚨
+
+**THIS MIGRATION REQUIRES MANUAL DATA UPDATE BEFORE DEPLOYMENT!**
+
+The migration adds a new constraint `remaining_tasks_state_consistency` that will **FAIL ON EXISTING DATA** if not handled properly.
+
+### Required Data Migration:
+
+Before applying this migration to any environment with existing data, you MUST include:
+
+```sql
+-- CRITICAL: Update existing step_states to satisfy new constraint
+UPDATE pgflow.step_states 
+SET remaining_tasks = NULL 
+WHERE status = 'created';
+```
+
+**Without this update, the migration WILL FAIL in production!** The new constraint requires that `remaining_tasks` can only be set when `status != 'created'`.
+
+---
+
+## Changes
+
+This patch introduces the foundation for map step functionality in the SQL core layer:
+
+### Schema Changes
+- Added `step_type` column to `steps` table with constraint allowing 'single' or 'map' values
+- Added `initial_tasks` column to `step_states` table (defaults to 1, stores planned task count)
+- Modified `remaining_tasks` column to be nullable (NULL = not started, >0 = active countdown)
+- Added constraint `remaining_tasks_state_consistency` to ensure `remaining_tasks` is only set when step has started
+- Removed `only_single_task_per_step` constraint from `step_tasks` table to allow multiple tasks per step
+
+### Function Updates
+- **`add_step()`**: Now accepts `step_type` parameter (defaults to 'single') with validation that map steps can have at most 1 dependency
+- **`start_flow()`**: Sets `initial_tasks = 1` for all steps (map step array handling will come in future phases)
+- **`start_ready_steps()`**: Copies `initial_tasks` to `remaining_tasks` when starting a step, maintaining proper task counting semantics
+
+### Testing
+- Added comprehensive test coverage for map step creation and validation
+- All existing tests pass with the new schema changes
+- Tests validate the new step_type parameter and dependency constraints for map steps
+
+This is Phase 2a of the map step implementation, establishing the SQL infrastructure needed for parallel task execution in future phases.