CVE-2026-59929: Mistune renderers/html.safe_url: HARMFUL_PROTOCOLS list misses legacy and chained schemes that historically chain to `javascript:` execution

Type: URL-scheme allowlist gap. The safe_url filter only blocks the four schemes javascript:, vbscript:, file:, data:. Several other schemes are accepted into rendered <a href="..."> and <img src="..."> tags despite being known XSS vectors in legacy or chain-handling browsers. The same gap applies to direct links, reference links, and autolinks. File: src/mistune/renderers/html.py, line 11-23 (HARMFUL_PROTOCOLS list). Root cause: the HARMFUL_PROTOCOLS tuple is a hardcoded, opt-out denylist of four entries. Browsers historically supported (and some still partially support) several other schemes that either execute JavaScript directly (livescript:, mocha:) or wrap a javascript: payload (feed:javascript:, view-source:javascript:, jar:javascript:, ms-its:javascript:, mk:@MSITStore:javascript:). On user-agents that still recognise these schemes (older Firefox builds for feed:/jar:, all Internet Explorer / Edge Legacy for ms-its:/mk:/res:, niche chrome-style browsers, browser extensions that register custom protocol handlers), clicking a link rendered by mistune executes attacker-controlled JavaScript in the page’s origin.