fix: handle markdown backslash escapes in EPUB, HTML, PDF, and DOCX exports

[rogerdigital]

Fixes #91

Problem

Escaped punctuation (\*, \|, \[, …) was not recognized by the basic markdown converter (markdownToBasicHtml) or the DOCX inline parser, so an escaped asterisk was treated as an emphasis delimiter:

• In EPUB tables (the reported crash): two \* in the same row paired into an <em> that crossed <td> boundaries, producing invalid XHTML — Apple Books fails with Opening and ending tag mismatch: em … and td. EPUB always uses this converter, so every export with escaped asterisks in tables was affected.
• Outside tables: \* rendered literally with its backslash; two escapes in one paragraph produced wrong italics plus a leaked backslash.
• Same class of issues: \| split a table cell in two, \[text\](url) still rendered as a link, \_/\\/\# kept their backslashes, and bare multiplication (3 * 4 * 5) was wrongly italicized (no CommonMark whitespace rules).
• DOCX had the same emphasis-pairing and pipe-splitting behavior (rendering errors only, no invalid XML).

Fix

• Extract backslash escapes into private-use placeholder sentinels before any syntax rule runs, restore them as literal characters afterwards — in markdownToBasicHtml and DOCX parseInline, sharing one exported MARKDOWN_ESCAPE_RE (CommonMark escapable punctuation). This alone fixes escaped asterisks, pipes, brackets, and all other escapes in both converters.
• Split table rows on unescaped pipes only (\| stays inside the cell) — basic converter table step and DOCX parseTableRow.
• Require non-whitespace content edges for bold/italic, per CommonMark delimiter rules, so 3 * 4 * 5 stays literal.

Restored characters are HTML-escaped like surrounding text (\< comes back as <). Code blocks and inline code are extracted before the escape pass, so their content is untouched.

Verification

• 12 new tests: 9 unit tests in html-document.test.ts (escaped */|/[/\, HTML-escape on restore, inline code untouched, multiplication boundary), 1 end-to-end EPUB test asserting balanced <em> tags in the chapter XHTML, 2 DOCX tests. All failed before the fix, pass after.
• Full suite: 425 tests passing; tsc type check, esbuild production build, and eslint clean.