feat: add script/count-blocked-by-build-error

[kim-em]

This PR adds a standalone Python script that wraps lake build, captures its output, and for each failed module reports how many modules in the selected lean_libs transitively import it. The count is an upper bound on how many downstream modules were blocked by the failure in the workspace.

The script enumerates modules via lake query <lib>:modules, builds the forward import graph using batched lake query +M:imports --json calls, and inverts it locally. A batched query that fails (e.g. because one module has an unparsable header) is bisected down to singletons so one bad file doesn't drop the whole report.

Example output:

$ script/count-blocked-by-build-error --lib Mathlib -- Mathlib.Topology.Basic
...lake build output streamed here...
Some required targets logged failures:
- Mathlib.Topology.Basic
error: build failed

Enumerating selected-library modules...
Building import graph for 5043 modules...

Selected-library dependents:
  Mathlib.Topology.Basic: 1842

This is a follow-up to #13454 (which added lake build --summary). The original issue #13316 asked for blocked-downstream counts inside Lake itself; moving that computation to a standalone script keeps Lake's build-reporting path minimal and avoids re-parsing every workspace module from within the build runner.

🤖 Prepared with Claude Code

[mathlib-lean-pr-testing]

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[leanprover-bot]

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