fix: bump and retry goose create on duplicate migration versions

[cestercian]

Bug

goose create can write two migrations with different names but the same version.

• Timestamp mode: two creates in the same second share 20060102150405, so later up panics in Migrations.Less (migrate.go) with a duplicate version. The provider path already returns a proper error.
• Sequential mode (-s): scan for last.Version+1 then create is TOCTOU. Concurrent creates collide; create itself can panic when the dir scan hits the duplicates.
• fix: return clear error when migration file already exists #1104 made create atomic with O_EXCL for the filename, but that does not protect version uniqueness across different names.

Fix

Option 2 from #1116:

• After the atomic O_EXCL create, detect when another file already owns that version.
• Bump the version (timestamp +1s, or sequential max+1) and retry O_EXCL.
• Exact same filename still returns os.ErrExist (keeps fix: return clear error when migration file already exists #1104).
• Sequential create no longer goes through collectMigrationsFS, so existing duplicate versions cannot panic create.
• timestampFormat is unchanged (no sub-second versions).
• Legacy CollectMigrations now returns an error for duplicate versions instead of panicking in Migrations.Less, matching provider_collect.go.

Tests

• Same-second timestamp collision (including later-name-first).
• Sequential collision / retry, existing duplicates, and concurrent creates.
• TestCreateDuplicateFile still expects os.ErrExist.

Fixes #1116

Drafted with Cursor; human-reviewed before opening.