v0.9.0: callouts, highlights, and richer Markdown rendering

md-bridge 0.9.0 teaches the renderer to speak more of the Markdown dialect it already declares. Five features land on the Markdown to PDF side, plus two opt-in additions on the PDF to Markdown side. Every change is additive: a document that does not use the new syntax renders exactly as before.

Added

• GFM alert callouts. > [!NOTE], > [!TIP], > [!IMPORTANT], > [!WARNING], and > [!CAUTION] render as a bordered box with an icon and a localized label (EN/PT/ES) instead of a plain blockquote. (#159)
• Custom containers. ::: warning … ::: blocks (the MkDocs/VuePress admonition syntax) render as the same callout box; the common names map onto the five types. (#164)
• Highlighted text. PDF text-highlight annotations become ==text== (opt-in extract_highlights), and ==text== renders to <mark> in the PDF. (#162)
• Strikethrough and task lists. ~~text~~ renders as <del>, and - [ ] / - [x] render as a disabled GitHub-style checkbox. (#143)
• Figure anchors. A numbered figure caption gives its image a stable {#fig-N} id for cross-references (opt-in emit_figure_anchors). (#165)

Notes

• No new dependency: every feature is a small python-markdown extension, so the default install stays lean. Each dialect addition is recorded as an amendment to ADR-001.
• Opt-in options (extract_highlights, emit_figure_anchors) default off, so existing conversions are byte-identical.
• Known gaps, tracked for later: == and callouts do not yet render in the DOCX output or the web preview (#412), highlights inside table cells are not extracted (#413), and table id anchors need a renderer that can round-trip them (#414).

Full changelog: v0.8.0...v0.9.0