feat: `lake build --summary` flag to print build outcome table

When building large projects, it would be useful to have a lake build --summary flag that prints a summary table at the end of the build, including:

• Number of files built successfully
• Number of files built successfully but with info and/or warnings
• For each file with an error, the number of downstream jobs that were not run as a result

This gives a quick at-a-glance picture of build health, especially useful for large projects where errors early in the dependency graph can cascade and block many downstream modules.

Example output (illustrative):

Build summary:
  ✔ 142 succeeded
  ⚠   3 succeeded with warnings
  ✖   2 failed
    Mathlib.Topology.Basic (blocked 27 downstream jobs)
    Mathlib.Order.Filter.Basic (blocked 14 downstream jobs)

🤖 Prepared with Claude Code

Start at the lake build entry point and trace how build jobs record successes, warnings, errors, and downstream jobs that are skipped. Define the summary table and its blocked-job counts, then verify the --summary output against builds containing successes, warnings, and dependency failures.