feat: AlterSchema, dropPredicate, and embedded DropAttr

[mlwelles]

What this adds

Raw schema-DDL primitives that complement UpdateSchema's struct-tag inference,
plus matching DropAttr support for the embedded engine.

• Client.AlterSchema(ctx, schema string) — applies a raw DQL schema string
  directly, giving full control over predicate types, indexes, and directives.
• Engine.dropPredicate — deletes a single predicate (and its data) from
  the embedded engine via posting.DeletePredicate.
• embedded routing — an Alter carrying DropAttr routes to
  dropPredicate, so the embedded (file://) path matches a remote cluster's
  DropAttr semantics.

The problem it solves

UpdateSchema infers the schema from Go struct tags, which is convenient but
cannot express predicates no Go type models yet — the common case during a
migration that adds or reshapes predicates ahead of the code. AlterSchema is
the escape hatch: hand Dgraph the exact DDL.

err := client.AlterSchema(ctx, `
    name:  string @index(exact) .
    email: string @index(hash) @upsert .
`)

Dropping a predicate had a second gap: a remote cluster honors an Alter with
DropAttr (delete one predicate and its data), but the embedded engine ignored
it. This wires the embedded path to posting.DeletePredicate, so a single-
predicate drop behaves the same in file:// and dgraph:// modes.

Tests

TestDropPredicateEmbedded runs the full lifecycle — declare → insert →
confirm present → DropAttr → confirm gone — against the embedded engine (and
against a live cluster when MODUSGRAPH_TEST_ADDR is set).

The automated review (cubic) found no issues.

Documentation

A runnable ExampleClient_alterSchema showing predicate types, indexes, and
directives applied directly.

[cubic-dev-ai]

No issues found across 4 files

_{Re-trigger cubic}