Explicit list syntax (_ul / _ol); bare lists become paragraphs

[connorferster]

Resolves the structural ambiguity where a YAML list under a heading meant either "several paragraphs" or "a bulleted list" depending on its contents.

Changes

• A bare list is now always a sequence of content items: strings → paragraphs, mappings → subsections. It is never auto-bulleted. This fixes the bug where a single wrapped paragraph under a heading rendered as a one-item bullet.
• Bullets/numbers are explicit via new _ul / _ol constructs, intercepted structurally in build_story (not registry blocks) so they compose with scoped text styles when those branches merge.
• Implicit detection (string-list → bullets, integer-keyed mapping → numbers) retired from dispatch; the check functions remain.
• convert_ol accepts a list (positional numbering, nesting) as well as a mapping (back-compat).

Breaking change

Pre-1.0 content break: documents relying on bare lists for bullets, or integer-keyed mappings for numbering, must adopt _ul / _ol. Migrates the test-data reports, all Examples, and the docs.

Testing

26 tests pass, incl. new tests/test_explicit_lists.py. Rendered checks confirm bullets/numbers/nesting appear and that previously mis-bulleted prose now renders as paragraphs.

Design doc: design/explicit-list-syntax.md.

Merge notes

Branched off main. Recommended to merge before features/scoped-text-styles (per intended order). Overlaps with the scoped-text-styles branch in story_builder.py / content_converters.py; expect a conflict on whichever merges second — the design doc notes where current_style threads into the _ul/_ol interception.

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