Constant MIGRATIONS 

pub const MIGRATIONS: &[MigrationFn];

Migration functions indexed by source version.

MIGRATIONS[i] upgrades the schema from version (i + 1) to (i + 2). For example:

• MIGRATIONS[0] upgrades v1 → v2
• MIGRATIONS[1] upgrades v2 → v3

Invariant: MIGRATIONS.len() == (STORE_SCHEMA_VERSION - 1) as usize (empty when STORE_SCHEMA_VERSION == 1, one entry when it’s 2, etc.)