Stamp nested blocks and list items with source lines - #263

Follow-up to #262.

What

Extends opt-in source-line tracking (sourceLines: true) from top-level blocks to nested blocks: content inside blockquotes, divs, list items, footnotes, and definition lists is now stamped with data-source-line too, along with <li>, <dt>, and <dd> nodes themselves.

echo (new DjotConverter(sourceLines: true))->convert("- first\n\n  second\n");

<ul data-source-line="1">
<li data-source-line="1">
<p data-source-line="1">first</p>
<p data-source-line="3">second</p>
</li>
</ul>

Why

Editor scroll-sync needs dense anchors. With top-level-only stamping, a document that is one long list or blockquote yields a single anchor, so sync inside it degrades to interpolation over the whole container. Now every item and nested block is an anchor.

How

• parseBlocks() / parseBlocksImpl() accept an optional per-line map (local index to 0-based document line, -1 for synthetic lines that must not stamp). Built at every extraction site: blockquote prefix stripping, div fences, list item/sublist gathering (including the synthetic blank-line separator), footnotes, definition lists. Maps compose through arbitrary nesting (list in quote, etc.).
• li/dt/dd are created outside the block dispatch loop and stamped where constructed. An empty <dd> (term without definition) anchors to its term's line.
• Raw HTML blocks and comments stay unstamped; author-set data-source-line attributes are never overwritten.
• Content parsed through the public parseBlockContent() helper (custom block callbacks) has no document positions and is mapped as unknown, so nested custom content is not mis-stamped with local indices (found by review).

Behavior / performance

• Option off (default): output byte-identical. The map bookkeeping is below measurement noise on a container-heavy ~1200-line document.
• Option on: same attribute path as before, denser stamping. No BC concerns (opt-in, attribute-only).

12 new tests (blockquotes incl. lazy continuation, divs, lists/sublists, synthetic separator, quote/list composition, footnotes, definition lists, author override, CRLF, custom blocks, default-off byte-identity).