html → markdown ignores <ol start> (storage → markdown honors it since #246)

[pchuri]

Follow-up from #243 / #247 and #241 / #246.

Description
#246 made storage → markdown honor <ol start="N">, but the html → markdown converter (lib/html-to-markdown.js, used by convert --input-format html) still ignores start and always numbers from 1.

Repro (v2.25.8)

htmlToMarkdown('<ol start="3"><li>a</li><li>b</li></ol>')

Actual:

1. a
2. b

Expected, matching storage → markdown since #246:

3. a
4. b

Notes

• Reuse the start parsing rules from fix(storage-walker): honor <ol start> in storage → markdown #246 (resolveListStart in lib/storage-walker.js): non-negative integers only, at most 9 digits, anything else falls back to 1.
• The continuation indent is already derived from the marker width, so 9. → 10. should indent nested content correctly.