Merge xVanTuring's GFM features + local bug fixes

[luca-chen198]

Summary

Re-introduces @xVanTuring's CommonMark/GFM feature contributions from his fork (https://github.com/xVanTuring/swift-markdown-engine/tree/main) together with local follow-up bug fixes.

The original merge was performed and pushed prematurely (commit 2ad2dee), then reverted (1988aac) so we could land it through a proper review workflow. This PR re-applies the same changes via a "revert the revert" commit. xVanTuring's original per-commit attribution is preserved in the git history (reachable through 2ad2dee).

From xVanTuring's branch (22 commits)

• Block syntax: GFM tables, blockquotes with nested bars, full-width thematic breaks, Setext headings
• Inline syntax: ~~strikethrough~~, _italic_ / __bold__, N-backtick code spans, CommonMark backslash escapes, ![alt](url) markdown image links
• Lists: marker normalization for -/*/+, space-indented nested bullets, backspace-restore on empty -
• ~8 rendering/styling fixes: emphasis-in-code containment, inline-code backticks active, table → child active propagation, code-block background scope, task-checkbox boundary restyle, wrapped block-image anchoring, image-provider fingerprint mutation, redundant NSValue cast

Local follow-ups (commit 76bad45)

• HR/blockquote: NSRangeException bounds-guard against TextKit's synthetic trailing line fragment (crash on Enter)
• HR: drop physical source expansion (was inserting 119 dashes); drop Setext support (was absorbing the last table row as H2 underline)
• HR: reveal raw source when caret enters the HR line
• Tables: skip inline LaTeX styling for $...$ inside a table — the stray .latexImage tag was drawing a tiny image as a dot on the collapsed source line
• Blockquote Enter: list-style continuation on Enter, exit on empty marker (mirrors bullet-list UX)
• Refactor: extract removeLinePrefixAndExit helper, shared by list-exit and quote-exit

Test plan

• GFM conformance sweep from xVanTuring's issue rendered correctly
• Tables: render correctness, caret in/out, LaTeX in cells, inline code in cells, no stray dot below
• Blockquotes: nested levels (>, >>, >>>), Enter continuation, Enter on empty exits without crash
• HR: all three forms (---, ***, ___); Enter on HR line doesn't crash; click on HR reveals raw source
• Strikethrough, underscore emphasis, backslash escapes
• Image links (![alt](url))
• Regression check: existing bullet/numbered lists still work; existing tasks ([ ] / [x]) still toggle
• Side-by-side with current main — no visual regressions in non-GFM cases