fix: clean up orphan build dirs from crashed runs

[aiolibsbot]

What

Sweep leftover `-tmpXXXX` directories (and `-build` symlinks) at the start of `make_index`, and stop the failure handler from masking the real exception.

Why

The atomic-replace flow in `_atomic_replace_old_index` does rename → symlink → replace. A crash between those steps leaves a fully-built `-tmpXXXX` directory in place forever — there is no GC anywhere. Worse, the existing `except` handler calls `rmtree(temp_dir)` on a path that was already renamed, raising `FileNotFoundError` and masking the original exception.

How

• New `_cleanup_orphan_build_dirs` runs while the lock is held, before `mkdtemp`. It removes siblings matching the build-name prefix, while preserving the one the live symlink points at.
• `rmtree(temp_dir, ignore_errors=True)` in the failure handler so the original traceback survives. Any orphan left behind is reclaimed by the next run's sweep.

Testing

• New `test_make_index_fails_partial_replace` simulates a crash after the rename; asserts the real exception propagates and the orphan is cleaned on the next run.
• New `test_make_index_cleans_orphan_build_symlink` seeds both an orphan dir and an orphan -build symlink; asserts both are gone after a successful run.
• Full suite: 16 passed.

🤖 Generated with Claude Code

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Quality Report

Changes: 2 files changed, 81 insertions(+), 2 deletions(-)

Code scan: clean

Tests: failed ([Errno 13] Permission denied: 'pytest')

Branch hygiene: 1 issue(s)

• Branch is not pushed to remote

Generated by Kōan post-mission quality pipeline