feat: let a labelled level drop its rows from General Statistics

[clintval]

Lets a labelled level fold its columns onto the group row without keeping its rows beneath it in General Statistics.

What changes

• Level gains keep_rows (default true, so existing configs render as before).
• With keep_rows: false, a labelled level's columns still fold onto the group row under their labelled titles, but the rows themselves leave General Statistics. Module sections are untouched, so the rows still appear there.
• keep_rows: false on a level without a label, or with a table, fails validation: those levels already remove or move their rows.
• Version 0.2.0.

Example

sample_pivot:
  group: '^(?P<group>[^.]+)\.'
  levels:
    - match: '\.subject$'
    - match: '\.(?P<tissue>tissueA|tissueB)$'
      label: '{tissue}'
    - match: '\.(?P<tissue>tissueA|tissueB) \(filtered\)$'
      label: '{tissue} (filtered)'
      keep_rows: false

Tested

• UV_NO_CONFIG=1 UV_PYTHON=3.12 uv run --locked poe check-all: format, lint, mypy and basedpyright clean; 31 tests pass at 100% coverage.
• New config, pivot and end-to-end tests cover the validation, the folded columns and the missing sub-rows.
• On a 12-subject cohort report, keep_rows: false on one labelled level removed its 24 rows from General Statistics while its module section still listed them.