Add PTB-XL dataset support (ECG)

[AxelNoun]

Contributor: AxelNoun (GitHub: @AxelNoun) — external contributor, no NetID
Contribution Type: New Dataset + Task
Description: Adds PTB-XL (12-lead ECG, PhysioNet v1.0.3, open access) as a
PyHealth dataset, with a 5-superclass multi-label diagnosis task and a
split helper using the official stratified folds. Motivated by the CPBench
effort: it gives conformal prediction a multi-label ECG benchmark that needs
no credentialing, with metadata suited to covariate/label-shift splits.
Based on Wagner et al., Scientific Data 2020
(https://www.nature.com/articles/s41597-020-0495-6).

Output label order: MultiLabelProcessor.fit sorts observed labels
alphabetically. On the full v1.0.3 corpus the multi-hot vector is
CD, HYP, MI, NORM, STTC. The task does not pin a custom vocabulary.

Files to Review:

• pyhealth/datasets/ptbxl.py — dataset implementation
• pyhealth/datasets/configs/ptbxl.yaml — table config (v1.0.3 template)
• pyhealth/tasks/ptbxl.py — 5-superclass multi-label task
• pyhealth/datasets/splitter.py — split_by_strat_fold helper
• tests/core/test_ptbxl.py — tests with synthetic fixtures
• docs/api/datasets/pyhealth.datasets.PTBXLDataset.rst, docs/api/tasks/pyhealth.tasks.ptbxl.rst
• examples/ecg/ptbxl/ptbxl_superclass_quickstart.py

Implementation notes:

• wfdb is an optional extra (pip install 'pyhealth[ptbxl]'), lazily imported.
• PTBXLDataset.root is the user-provided data path. Derived metadata CSVs
  live under metadata_cache_dir (not root) so read-only mounts stay
  untouched. BaseDataset.load_table concatenates root / file_path and
  cannot take an absolute path, so load_table is overridden to scan the
  cache directory. Cache identity still hashes data-root path + source CSV
  bytes into dataset_name (EEGBCI pattern) so two roots or an in-place
  ptbxl_database.csv replace cannot share global_event_df.
• Resolved cache YAML: BaseDataset only accepts config_path, so PTB-XL
  writes a rate/root/source-keyed YAML next to the derived CSV before
  super().__init__(). CSV and YAML writes use tmp+os.replace under
  filelock.FileLock.
• Unrelated pre-existing # noqa: E402 on the EEGBCI import line in
  pyhealth/datasets/__init__.py — left untouched (out of diff).
• Signals returned as (n_leads, n_samples), matching EEGBCI's convention.
• Sample age is always an int: missing → -1 (age_is_missing=True);
  HIPAA-censored raw 300 is clipped to 90 (age_is_censored=True). The
  raw 300 remains in the derived metadata CSV.
• Empty superclass sets (≈400 records on v1.0.1, Wagner et al. Table 9;
  mainly pacemaker ECGs) are dropped by default via drop_empty_labels.
• split_by_strat_fold(..., folds=) avoids decoding 12-lead signals just to
  read an integer. check_patient_disjoint=True raises if a patient_id
  appears in more than one split (official PTB-XL folds are patient-disjoint;
  the synthetic fixture leaks patient 15709 across folds 1 and 9 to exercise
  this).
• Fixture under test-resources/core/ptbxl/ is synthetic CSVs; WFDB
  waveforms are generated at test time (not committed).

Testing / CI:

• Local: python -m unittest discover -t tests -s tests/core -p 'test_ptbxl.py' -v — 28 passed (with wfdb installed).
• Without wfdb / pyhealth[ptbxl], waveform I/O and set_task e2e tests skip.
• pixi test feature uses pyhealth = { extras = ["ptbxl"] } so make testall under pixi covers waveform I/O.

Follow-up: 71-SCP-code classification and age regression are structured
for but not implemented here — planned as a separate PR.

Test plan

• python -m unittest discover -t tests -s tests/core -p 'test_ptbxl.py' -v — 28 passed (with wfdb)
• python tools/check_pr_rules.py --base upstream/master --head HEAD
• CI make testall / contribution-rules on the PR

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Recreates #1186, which was closed automatically on 12 Aug when the fork
hosting its branch was detached during an account cleanup on my side —
my mistake, apologies for the noise.

All prior context and discussion: #1186