Ensure Deterministic Migration Order

[aoengin]

What / why

When embedding migrations into the binary via migrate!, the iteration order of migration files can vary across filesystems/OSes. That makes the resulting embedded migration list (and thus the final binary) non-deterministic, which breaks reproducible builds.

This PR makes migration ordering deterministic by explicitly ordering reversible migration pairs so that up is always before down for the same migration version.

Changes

• Add MigrationType::direction_order() to define a stable tie-break rule (up/simple first, down second).

• Update Migrator::with_migrations() to sort migrations by:

  1. version ascending
  2. then direction_order() for version ties.

• Apply the same deterministic sort when resolving migrations from a source (blocking resolver).

Impact

• Not a breaking change: applied migrations and version ordering remain the same; only the deterministic ordering of reversible pairs is enforced.
• Fixes non-determinism in builds that embed migrations.