Module migration 

Schema migration types and traits.

The migration subsystem lets compiled #[derive(Table)] schemas evolve across releases without manual stable-memory surgery. The flow is:

1. On boot, the DBMS compares the hash of every compiled TableSchemaSnapshot against the hash stored in the schema registry.
2. If they differ, the DBMS enters drift state and refuses CRUD.
3. The user calls Dbms::migrate(policy), which diffs the stored snapshots against the compiled ones and produces a Vec<MigrationOp>.
4. The ops are applied transactionally; on success the new snapshots and schema hash are persisted and the drift flag is cleared.

This module owns the types (MigrationOp, ColumnChanges, MigrationPolicy, MigrationError) and the per-table extension hook Migrate. The diff algorithm and apply logic live in the engine crate.

Structs

ColumnChanges

Bundle of constraint-flag deltas for an MigrationOp::AlterColumn.

MigrationPolicy

Caller-supplied policy that gates destructive migration ops.

Enums

MigrationError

Error variants produced by the migration planner and apply pipeline.

MigrationOp

Single atomic step produced by the migration planner.

Traits

Migrate

Per-table extension hook for schema migrations.