EF Core sweep follow-ups: sequences, check constraints, computed columns, index methods, drift detection, Oracle/MySql parity

[jeremydmiller]

Follow-up round to #361, working through the remaining gaps the schema-parity sweep documented.

What's here

• Sequences — SequenceBase.IncrementBy (HiLo's block size is load-bearing), a Migrator.CreateSequence seam (PG / SQL Server / Oracle; MySQL & SQLite return null), and GetSchemaObjectsForMigration mapping model sequences (UseHiLo, UseSequence, HasSequence) ahead of the tables. CreateMigrationAsync now includes them. SQL Server's UseSequence NEXT VALUE FOR defaults flow through and match.
• Check constraints — TableCheckConstraint + ITable.AddCheckConstraint, CREATE TABLE emission, catalog reads, and delta comparison on PostgreSQL and SQL Server. Deliberately conservative: only constraints the model declares are compared, and constraints Weasel doesn't know about (inline column checks, third-party) are never dropped.
• Computed columns — ITableColumn.ComputedExpression/ComputedColumnIsStored, emitted on PG (GENERATED ALWAYS AS ... STORED), SQL Server (AS (...) [PERSISTED]), MySQL, and SQLite (mapped onto its existing generated-column model). Oracle throws rather than silently emitting a plain column.
• Index methods — ITableIndex.Method; Npgsql HasMethod("gin") now maps automatically (no more escape hatch for gin).
• Opt-in column drift detection — ITable.DetectColumnDrift: default-expression + nullability comparison for otherwise-matching columns with ALTER COLUMN corrections on PG and SQL Server (incl. dropping SQL Server's server-named default constraints). Off by default — datetime literal canonicalization isn't stable enough to make it safe blanket-on, and a false positive means perpetual re-apply. Direct provider tests cover changed/dropped defaults and nullability drift round-trips.
• Oracle casing parity — the same quoted-identifier treatment PostgreSQL got in EF Core mapping sweep: schema parity with EF Core migrations (tables + indexes) #361, honored by PreserveIdentifierCase (folded convention byte-identical), covered by DB-free DDL tests (live verification rides on ci-build-oracle; the arm64 local container won't boot).
• MySQL in EF CI — service added to ci-build-efcore.yml, MySql filter removed (net9.0), EnsureDeleted hazard removed from its e2e tests. Verified locally against mysql:8.0.
• Also: SQL Server FetchExisting now reads column defaults/nullability, SS ItemDelta matching is case-insensitive, and the EF docs (docs/efcore/table-mapping.md) are updated for casing/sequences/owned types/drift.

Verification

• EF suite: 64/64 (net9.0) + 62/62 (net10.0) — including 5 new comparison suites (PG sequences, PG checks+computed, SS sequences, SS checks+computed)
• Weasel.Postgresql.Tests 767 ✅ (5 new drift tests), Weasel.SqlServer.Tests 302 ✅ (3 new drift tests), Weasel.Sqlite.Tests 361 ✅, Weasel.Core.Tests 21 ✅, Oracle DDL unit tests 4/4 ✅ (one pre-existing env-only failure needing live Oracle, unchanged on clean master)

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