Editor: add paragraph-level editing support

[tmustier]

Problem

pptx edit only supports single-paragraph editing. Multi-paragraph text (separate lines with different formatting) cannot be created or preserved.

Current behavior

# Runs array → merged into single paragraph
pptx edit file.pptx 2 16 '[{"text": "line1"}, {"text": "line2"}]'
# Result: "line1line2" on one line

# Paragraphs format → clears text
pptx edit file.pptx 2 16 '{"paragraphs": [[{"text": "line1"}], [{"text": "line2"}]]}'
# Result: empty text

Expected behavior

pptx edit file.pptx 2 16 '{"paragraphs": [[{"text": "line1"}], [{"text": "line2", "bold": true}]]}'
# Result: two lines, second one bold

Root cause

_normalize_content() in editor.py only handles run-level content. The {"paragraphs": ...} format is not recognized and falls through to empty result.

Workaround

Use soft line break (\n which becomes \x0b) for multi-line text within a single paragraph:

pptx edit file.pptx 2 16 $'line1\nline2'

This works but all lines share the same formatting.