feat: add _p block for standalone styled paragraphs

[connorferster]

Summary

Adds a new _p block that emits a standalone paragraph, so an author can write body text after another block without first introducing a heading key (and without a bare list item, which gives no control over the text style).

Two value forms:

- _p: A quick paragraph in the default style.     # bare string

- _p:
    content: Small print after a figure.          # mapping form
    style: fine-print                             # any _style.styles family

• content holds the text; text is accepted as an alias.
• style selects a text style family (the same named families as _style.styles / _textstyle), rendered in that family's body role. Defaults to default. An unknown style raises YMPrintSyntaxException listing the available families.
• Reuses convert_paragraph, so inline markdown and {{variable}} interpolation work as in ordinary body text.

Implementation

• src/ymprint/blocks/p_block.py — the converter plus register_block("_p", …). Family validation is inlined to avoid a circular import (story_builder imports the block registry).
• Registered in report_reader.py alongside the other blocks.

Tests

tests/test_p_block.py — 10 tests covering all value forms, the text alias, named-family styling, error paths (unknown style, missing content, invalid value type), markdown/Jinja rendering, and an end-to-end build_story case. Full suite: 76 passed. Also verified end-to-end with a real ym convert render.

Docs

• docs/reference/blocks.md: new (block-p)= section and catalogue row.
• README.md: _p row in the blocks table.

Note

docs/reference/blocks.md contains a pre-existing, committed git merge-conflict marker (<<<<<<< HEAD … >>>>>>>) around the catalogue table — unrelated to this change. I placed the new catalogue row above it and left the conflict untouched; it should be resolved separately.

🤖 Generated with Claude Code

https://claude.ai/code/session_014MCYhna7e6jhcunu3DCT6j