…ention edge cases

- CommonMark allows up to 3 leading spaces before a block-HTML opening line; the new block tokenizer required column 0, so an indented <details>/<div> still fragmented across blank lines. Fixed by splitting off the leading indent, matching against the rest, and stitching the indent back onto raw.
- The open-tag and balanced-close regexes stopped at the first `>`, even inside a quoted attribute value (e.g. data-example="a > b"), producing wrong match lengths and miscounting a same-tag mention inside the quoted value as a real nested tag. Replaced with an attribute-aware pattern that treats a full quoted value (including any interior >) as one unit.
- The balanced-tag scan couldn't distinguish real markup from a tag name mentioned inside inline code or a fenced code block. Now masks code regions (same-length filler, positions preserved) before scanning, so a properly-escaped mention (backticks or a fenced example) is never miscounted. A genuinely bare, unescaped mention outside code remains a known, inherent limitation of regex-based tag matching (shared by real HTML parsers given the same ambiguous input) — verified this can never lose data or hang, only reflow to a stable fixpoint on save.

…ce scan

- maskCodeRegions's fenced-code regex required fence markers at column 0, so a fence quoted inside a markdown blockquote (each line prefixed with `> `) wasn't recognized, leaving tag mentions inside it visible to the balance scanner. Extended the fence pattern to tolerate an optional blockquote prefix on both the opening and closing fence line.
- BLOCK_HTML_TAG_NAMES includes several void elements (link, meta, base, hr, ...) that have no closing tag at all. The block tokenizer only treated an explicit self-closing `/>` as complete, so a void tag without one would scan the rest of the document for a `</meta>` that will never appear, risking a false match on a later same-name mention. Now reuses the existing VOID_TAGS set (already used by the inline tokenizer) to treat these as complete right after the open tag. Also restored `hr` to the whitelist, which was accidentally dropped when transcribing marked's tag list.