Add rich clipboard (CF_HTML build/parse/get/set)

[JE-Chen]

The base clipboard handles plain text + image only; rich paste into Word / Outlook needs CF_HTML, whose byte-offset header (StartHTML/EndHTML/StartFragment/EndFragment) is famously error-prone.

• build_cf_html(html) → valid CF_HTML UTF-8 bytes whose offsets point exactly at the fragment (fixed-width 10-digit header → single-pass offsets; correct across multi-byte UTF-8). parse_cf_html(blob) recovers the fragment from bytes/str (comment markers, falling back to byte offsets). Both pure-stdlib and fully round-trip tested.
• set_clipboard_html / get_clipboard_html: Win32 wrappers (RegisterClipboardFormat "HTML Format" + the GlobalAlloc/SetClipboardData pattern from the base clipboard module); raise RuntimeError off Windows, with a plain-text fallback via fragment_plaintext.
• The error-prone byte-offset math is headless-tested; the Win32 I/O guard is verified on Linux CI. (CF_HDROP file-list deferred — fiddly OLE, not headless-testable.) Wired through all 5 layers + headless test + EN/Zh docs + WHATS_NEW. Qt-free.

[codacy-production]

Not up to standards ⛔

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud