Restructure CI/CD pipeline: consolidate workflows, reorganize tests, fix PyPI publishing

[anth-volk]

Problem

• 9+ workflow files, many deprecated or broken
• Versioning workflow broken — PAT expired, preventing PyPI publishing
• Tests disorganized — unit and integration tests mixed together, some tests in wrong directory and never discovered by pytest
• No test coverage reporting
• Integration tests run on every PR (~3 hours) even for doc-only changes

Solution

Fix PyPI publishing

Migrate versioning workflow from expired PAT to GitHub App token.

Reorganize tests

Move all tests to top-level tests/ with unit/ and integration/ split. Integration tests named per-dataset. Merge sanity tests into per-dataset files. Fix broken tests. Remove test_reproducibility.py (imported nonexistent modules since July 2025).

Consolidate workflows

Replace 9 workflow files with 4:

• pr.yaml — Fork check, lint, uv.lock, changelog, unit tests with Codecov, smoke test. Integration tests only with run-integration label.
• push.yaml — Single-container Modal data build with integration tests, manual approval gate, pipeline dispatch. PyPI publish on version bumps.
• pipeline.yaml — Dispatch-only H5 generation with scope filtering.
• versioning.yaml — Auto version bump with GitHub App token.

Add Codecov

Informational coverage reporting on PRs (non-blocking).

Prerequisites (manual setup after merge)

• Create pipeline-approval GitHub environment with required reviewers
• Add CODECOV_TOKEN secret from codecov.io