feat(migrate): revision-chain ordering + struct-snapshot scaffolding

[mlwelles]

Summary

Adds Alembic-style explicit revision-chain ordering and struct-snapshot migration scaffolding to the migrate engine, migratecli, and modusgraph-gen.

Ordering — the revision chain

• Migration.After names each migration's predecessor; exactly one root has After == 0. The runner walks the chain instead of sorting by ID, so it catches parallel-branch divergence a timestamp sort hides.
• buildChain validates before any work and returns a typed error on the first fault: ErrDuplicateID, ErrNoRoot, ErrMultipleRoots, ErrUnknownPredecessor, ErrDivergentHistory, ErrCycle.
• up/down/status now order by the chain. After is folded into the migration checksum (structural → immutable once applied).
• migrate history [--tree] [--verbose] renders the chain, marks forks, and exits non-zero on a structural fault — a CI chain lint.

Scaffolding

• modusgraph-gen emits schema.Models() []any (one zero value per entity), so tooling cannot fall out of sync with the declared types.
• schema_state.schema is a checked-in, tool-managed desired-state snapshot.
• diffSchema classifies each predicate change: Added/IndexChanged → emitted as an additive EnsureSchema delta; TypeChanged/Removed → flagged for RetypePredicate/explicit drop, never auto-applied.
• Scaffold / Snapshot / Diff write the <id>_<name>.{go,schema} pair, advance the snapshot, and AST-append the new var to the All slice (best-effort). ResolveDir fails fast with actionable messages outside a project root.
• Verify checks the live schema against the structs (one-directional); detects Missing everywhere and Mismatched against a real Dgraph.

CLI + docs

• migratecli: create / diff [--check] / snapshot / verify commands; Provider gains Models(). diff --check and verify exit non-zero on drift.
• New migrate/MIGRATIONS.md library guide + runnable Example* tests.

Testing

go build ./... && go test ./... — all packages pass.