Parallelize block comparison itself

[hittjwX]

• See TidyObsidian/find-duplicate-blocks.py

This script takes 30-40 minutes to process 22K files. Currently, only reading and block extraction are parallel; step 5 runs in a single process.
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A simple structural improvement:

• Split all_blocks into chunks and run the “for each block, find candidate indices, check Jaccard, append or merge” logic in worker processes.

• Have each worker build its own local canonical_blocks/token_index, then merge the results at the end (e.g., by re-running a cheaper global dedup on the worker outputs).

This is more work to implement, but if 30–40 minutes is mostly CPU, using all cores in step 5 can cut that substantially.