[FIX] Forward the extraction thread count to the integration-test notebook

[noel-improv]

Description

build-tests.sh writes the extraction settings a run needs into the notebook's .env.testing. #490 added EXTRACTION_NUM_WORKERS and EXTRACTION_BATCH_SIZE there, but not EXTRACTION_NUM_THREADS_PER_WORKER, so every run extracted at the default of four threads regardless of what was set locally. This forwards it the same way as its siblings and documents it in env.template.

Changes

• integration-tests/build-tests.sh: write EXTRACTION_NUM_THREADS_PER_WORKER to .env.testing when it is set, using the same printf %q form as the settings next to it.
• integration-tests/env.template: add EXTRACTION_NUM_THREADS_PER_WORKER with its default of 4.

Problem

A thread or worker sweep cannot vary the setting it is measuring. It showed up on the single-worker drain re-run for #493, which needed 32 threads and would otherwise have run the 4-thread arm.

Related issue (if any): #493

Testing

• Unit tests added/updated
• Integration tests added (as appropriate)
• Existing tests pass (pytest)
• Tested manually (describe below)

bash -n passes. Running the new block on its own writes export EXTRACTION_NUM_THREADS_PER_WORKER=32 when the variable is set and writes nothing when it is unset. The same line carried a real run: a stack launched with EXTRACTION_NUM_THREADS_PER_WORKER=32, and the notebook's shipped .env.testing contained that value.

Checklist

• Code follows existing style and conventions
• License headers present on new files
• Documentation updated (if applicable)
• No breaking changes (or clearly documented)

No new files. Unset, the script behaves as before.

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