fix: validate chunk CRCs when reading canonical episodes after sync

[mulkakhileshmj]

Closes #474.

Problem

Every post-sync read of a canonical episode ran with chunk CRC validation off. Episode._reader opened the file through open_reader(path), whose default is validate_crcs=False, and the META lane, the relabel lane, and the online re-check flow all consume that reader. A canonical episode that decayed on disk after sync was therefore re-read and re-certified: checks stamped fresh measured findings over bytes that fail the file's own integrity stamp, and those rows are what curation and export trust.

Fix

Episode._reader now opens with validate_crcs=True. On a damaged episode every data-reading check records a diagnosed CRCValidationError per episode instead of fresh evidence, so the catalog never certifies the file and default_dataset_sql never ships it. A healthy canonical runs the check lanes unchanged, and the CRC pass rides on decompression that was already happening (#462 carries the cost measurement).

I audited the other open_reader call sites: app.py (sync-reuse witness), importers/lerobot.py, and runtime/_templates.py read only summary and metadata records, which chunk CRCs cannot vouch for, so they keep the default. The open_reader docstring now states that rule instead of the "already identified by content hash" claim the issue debunked.

No TRANSFORM_BEHAVIOR_VERSION bump: the transform writes identical bytes, only reads changed.

Coverage

New regression test in tests/test_processing_regressions.py following the issue's repro: process a synthesized episode, flip one bit of the first chunk's stored uncompressed_crc (helper added to tests/reuse_test_helpers.py beside the existing corruption helper), and assert the META re-run reports content_digest as ERROR with the CRC diagnosis and stamps no fresh measurements, while a healthy control run stays fully MEASURED.

Validation

Run on WSL2 Ubuntu, Python 3.12:

uv sync --locked
uv run ruff check          # All checks passed
uv run ruff format --check # already formatted
uv run ty check            # All checks passed
uv run pytest -q           # 1712 passed, 6 skipped

Mutation check per the issue's definition of done: with the one-line fix reverted the pinning test fails; restored, it passes. The touched test file also passes under Python 3.11.

[github-actions]

👋 Hi @mulkakhileshmj — thanks for the contribution! To keep starter issues available
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You already have #476 open, so this one is being closed automatically.
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