feat(core): workflows with job dependencies (M8)

[giraffesyo]

First of the M8 PRs: DAG workflows. Streams and hopperui follow separately.

What changed

• API: hopper.NewWorkflow(name, opts), wf.Add(args, opts) Step, hopper.After(steps...), client.InsertWorkflow/InsertWorkflowTx, client.WorkflowGet, and hopper workflows get <id>. A step can only depend on steps added before it, so a workflow is a DAG by construction. StepOpts.OnDependencyFailure is DependencyCancel (default) or DependencyIgnore.
• Model: a workflow is a batch whose jobs depend on each other, so it gets the batch's progress and callbacks for free (OnSuccess/OnFailure/OnComplete), a name, and its graph on the batch row. Steps with dependencies are inserted pending; roots are available at once.
• Schema v4: hopper_job_deps (the working set of unsatisfied edges), hopper_batches.name and .edges, and a batch_id index on history for inspection.
• Finalize: every statement that finalizes jobs (batched finalize, cancelling a waiting job, expiring) now produces a finished CTE in one shape and ends with the same tail: cascade-cancel the pending dependents of a failed job (recursive CTE, cancel edges only), archive, count batches down and insert callbacks, promote dependents whose dependencies have all left the live table, notify their queues, drop the satisfied edges. This replaced the separate history-insert and batch-accounting helpers.
• Driver: WorkflowInsert (IDs generated up front by the database, batch + jobs + edges in one transaction, no unique keys allowed) and WorkflowGet; DependencyFailure, JobDependency, JobEdge, WorkflowRow.
• Tests: drivertest Workflows (diamond promotion, transitive cascade with an ignore branch, cancelling a pending step, validation) runs on both drivers; a client-level end-to-end test; a CLI case.
• Docs: plan §4.6, §6, §11, §13, §16; changelog; getting-started section on batches and workflows.

hopperbench

Local, PG 17, -jobs 60000, five rounds alternating base (feat/hoppersql) and head, median run:

insert       base 185729 jobs/s -> head 194540 jobs/s (+4.7%) ok
latency      base p50 1.00ms p99 2.36ms -> head p50 1.01ms p99 2.46ms (-4.0%) ok
retry        base 31578 jobs/s -> head 32292 jobs/s (+2.3%) ok
throughput   base 68895 jobs/s -> head 69180 jobs/s (+0.4%) ok

The finalize tail adds index probes on hopper_job_deps per finalized job, which is empty for plain jobs; no measurable cost.