fix: preserve literal Markdown inside inline code

[xiaoheiai4719-collab]

Summary

Keep inline code out of the emphasis and strikethrough replacements, then restore it unchanged. This prevents examples such as **literal**, __name__, a*b*c, and ~~literal~~ from being formatted inside code spans.

The existing escaping and single-backtick matching remain in place. Formatting around code spans still works. Regression tests cover multiple spans, delimiter boundaries, HTML escaping, and literal placeholder-like text.

Validation

• Baseline with the new parser specs: 21 failed, 8 passed
• Fixed parser specs: 29 passed
• Six utility spec files: 112 passed with Jasmine under Node
• Frontend production build and spec TypeScript check passed
• ChromeHeadless test bundles compiled, but Chromium could not launch because this environment blocks its local socket creation. Browser execution still needs verification

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