html → markdown turns <br/> in list items and table cells into a space (diverges from storage → markdown)

[pchuri]

Follow-up from #242 / #248.

Description
After #248, storage → markdown keeps <br/> inside list items as a hard line break, and inside table cells as <br>. The html → markdown converter (lib/html-to-markdown.js) still turns both into a space, so the two converters now disagree on the same input. #149 was about exactly this kind of drift.

Repro (v2.25.8)

List item:

htmlToMarkdown('<ul><li>line1<br/>line2</li></ul>')
// html:    "- line1 line2"
// storage: "- line1\\\n  line2"   (storageToMarkdown on the same markup)

Table cell:

htmlToMarkdown('<table><tbody><tr><td>line one<br/>line two</td></tr></tbody></table>')
// html:    "| line one line two |\n| --- |"
// storage: "| line one<br>line two |\n| --- |"

Expected
Both converters should produce the same output as storage → markdown.

Notes

• fix(storage-walker): keep <br/> inside list items as a hard line break #248 added the HARD_BREAK sentinel (U+E003) in lib/markdown-cleanup.js, plus escapeContinuationLine and trailing-backslash handling in lib/storage-walker.js. Reuse these rather than duplicating them. Watch for the edge cases fix(storage-walker): keep <br/> inside list items as a hard line break #248 covered: a trailing \ before the break, and continuation lines that look like block syntax (- , 1. , #, --- | ---).
• Top-level <br/> is a soft break in both converters. It is out of scope here.