Add opt-in Discord-style __underline__ rendering

[PiotrCzapla]

What changed

New underline option (off by default). When enabled, Discord-style __x__ renders as <u>x</u> instead of <strong>x</strong>:

to_xhtml("__important__", underline=True)   # <p><u>important</u></p>

Also exposed as xhtmlmd --underline on the CLI and Options::underline in Rust. Callbacks get a new underline inline kind.

Behavior details

• Only underscore runs of length 2 are affected; **x** stays <strong>, _x_ stays <em>.
• All CommonMark underscore emphasis rules still apply: no intraword matching (intra__word__ stays literal), delimiter-run pairing is unchanged, and ___x___ nests as <em><u>x</u></em> — which matches Discord's underline-italics.
• ____x____ flattens to a single <u> the same way nested empty-attr <strong> already flattens.
• Trailing {.class} span attributes are not applied to __x__, consistent with existing **x**/_x_ behavior (bracketed spans are the attribute mechanism).

Why opt-in

CommonMark defines __x__ as strong emphasis, and the crate's dialect policy is Pandoc-closest. Making this the default would deviate from every emphasis spec example using __; as an option it costs nothing when off — the full conformance suite passes with no examples disabled.

Implementation

Inline::Underline is a new AST variant mirroring Strong; the underline flag threads from Options through delimiter processing, and the _-with-length-2 case in wrap_delimiters picks the variant. Renderer emits <u>, and the Python node/callback plumbing treats it like the other child-bearing inlines.

Testing

• New tests in tests/test_focused.py: opt-in gating, coexistence with **/_, nesting/flattening/intraword rules, and the underline callback.
• Full suite: 820 passed; the only failures are the two tests importing machine-local packages (fastpylight, math_core). tools/gen_docs.py --check is clean. README and docs/DIALECT.md updated.

🤖 Generated with Claude Code

[jph00]

I'm open to this, although my preference would be to use ++ instead, since that's a known markdown-it extension and doesn't require a new option. Other possibilities that work now are [x]{.underline} (Pandoc's HTML writer special-cases that class and emits <u>) or just Raw <u>x</u>

WDYT?