feat: check that migrations are ordered

[ilbertt]

Migrations apply in filename order, and that is the order you meant only while every filename carries a sequence prefix of the same width — 1, 2, 10 applies as 1, 10, 2, silently building a schema production never had.

Naming a prefix pattern is what turns the check on — in the config, or as the flag's value:

// sqlgen.config.ts
export default defineConfig({
  checkMigrationOrder: { prefixPattern: /^\d+/ }, // 0001_init.sql, 0002_add_users.sql, …
});

bun bun-sqlgen generate 'src/**/*.ts' --migrations db/migrations --check-migration-order '^\d+'

There's no default pattern: only the project knows which convention its filenames were named for, and a wrong guess passes a check that never looked at the right prefix.

Width, not "is it a number", is what's checked: equal-width prefixes sort the same way in any positional scheme, so a scheme that doesn't number at all is held to its own terms (/^[a-z]{4}_/, /^\d{14}_/). It fails on a migration with no prefix, on two claiming the same prefix (0035 twice), and on prefixes of differing widths. Gaps are fine.

Deliberately not folded into --check: unlike the other checks it guards generation itself rather than the output, so it runs in every mode — and folding it in would break CI for projects with unprefixed migrations that work fine today.