[rr-]

Compose a blank line inside an item's description as an empty line, not as a line of indent spaces, so that compose(parse(text)) gives back the text of #61 exactly and composed docstrings have no trailing whitespace.

• Google, Numpydoc and Epydoc joined the description lines with the indent, so every blank line got it too. They now go through a shared hang() in _compose.py, which indents only the lines that have text. ReST already wrote blank lines empty, and the other places that indent text (sections, directives, attribute bodies) already skipped them.
• test_invariants.py gets test_compose_writes_no_blank_line_with_indent: every case input whose own blank lines are empty composes, in every style and rendering, with no line of spaces alone. It fails on main for Google, Numpydoc and Epydoc. test_google.py adds the example from newlines not preserved in google param description #61.
• just update-cases rewrote 54 case files; all 123 changed lines are compose output where a line of spaces became an empty line.
• Parity against main: 2430 of 29184 docstrings differ, all in compose output (case 1317, stdlib 204, generated 28179 outputs), and every one is the same text once lines of spaces alone are emptied. No parse output changes.
• Bench: worst ratio 1.01 on Python 3.8 and 1.04 on 3.14.

Closes #61.