Add a dv flow that bundles child configs of different flows

[martin-velay]

Part of lowRISC/dvplan#129, and the point of the whole exercise on this side. Depends on the source-naming, formal-annotation and per-child-tool issues.

One command should run a block's simulation regression and its formal regression and score one vPlan from both. Something like:

{
  flow: dv
  name: hmac_dv
  vplan: "{proj_root}/hw/ip/hmac/data/hmac_vplan.hjson"
  dut_instance: "tb.dut"
  use_cfgs: [
    { cfg: "{proj_root}/hw/ip/hmac/dv/hmac_sim_cfg.hjson",  tool: xcelium  }
    { cfg: "{proj_root}/hw/ip/hmac/fpv/hmac_fpv_cfg.hjson", tool: vcformal }
  ]
}

Most of the machinery is there. _load_cfg resolves each child's class from that child's own flow key, so mixed children are already constructible, and deploy_objects gathers jobs from every child. What blocks it is that each primary class assumes its children share its own type, such as the TypeError in OneShotCfg.gen_results.

Scope

• A primary-only config class for flow: dv. It has no build or run jobs of its own, requires use_cfgs, and refuses to be a child of another primary config.
• It owns the single vPlan annotation job, which depends on every child's terminal jobs and reads the concatenation of every child's vplan_sources(). One invocation of DVPlan, because whether an item is unmeasurable is judged over the whole set of sources a run is given.
• A results summary holding each child's own table plus the plan's score. Children keep their existing per-flow reports.
• Command line behaviour on a bundle needs deciding and documenting: reject --tool, since it cannot mean one thing across two flows, and settle what -i, --cov, --build-only and --purge mean when they reach children of different flows. Rejecting with a clear message is a fine first answer for any of them.
• A child naming its own vplan while the bundle names one is a conflict, so warn and let the bundle win.

Done when

• A bundle config runs both children and writes one annotated plan scored from the simulation report, the formal report and the evidence file.
• Each child still runs standalone, unchanged.
• The unsupported command line combinations fail with a message that names what to do instead.