Broaden fenced-code-block compatibility for real-world Markdown

[adrianbrowning]

What to build

Broaden Markdown fenced-code-block support so mdcode works predictably with common documentation conventions instead of only a narrow fence subset.

Acceptance criteria

• The parser supports both backtick and tilde fenced code blocks.
• Opening fences with three or more matching fence characters are handled correctly.
• Indentation and closing-fence behavior is documented and tested against representative Markdown inputs.
• Unsupported forms fail or are ignored predictably without corrupting surrounding Markdown.
• Existing supported documents remain backward compatible.

Blocked by

None - can start immediately

[adrianbrowning]

This was generated by AI during triage.

Agent Brief

Category: enhancement
Summary: Support CommonMark-style backtick and tilde fences without changing existing documents.

Current behavior:
The parser recognizes only three- or four-backtick opening fences. Its closing-fence and metadata-update scans duplicate that narrow rule, so longer fences and tilde fences are ignored or cannot be safely rewritten.

Desired behavior:
mdcode recognizes common fenced code blocks predictably and preserves their original fence form when it updates Markdown. Unsupported or incomplete fences remain ordinary surrounding Markdown rather than becoming partial blocks or causing unrelated rewrites.

Key interfaces:

• Opening fence — accept zero to three leading spaces, followed by either backticks or tildes, with a fence length of at least three.
• Closing fence — require the same fence character, a length at least as long as the opening fence, zero to three leading spaces, and only trailing whitespace.
• Info string — reject a backtick opening fence whose info string contains a backtick; permit it for tilde fences.
• Incomplete/invalid fence handling — do not yield a block or rewrite surrounding Markdown.
• Rewrite preservation — updates preserve the original opening fence character, length, and indentation.

Acceptance criteria:

• Both backtick and tilde fences parse correctly.
• Opening fences with every length of three or greater parse correctly.
• Closing fences require the matching character and a sufficient length.
• Indentation and trailing-whitespace rules follow the documented contract.
• Backtick info strings containing a backtick are rejected; equivalent tilde fences are accepted.
• Unterminated or invalid fences are ignored predictably without corrupting surrounding Markdown.
• Updating parsed blocks preserves each original fence form.
• Existing supported three- and four-backtick documents remain compatible.
• Parser and rewrite tests cover representative valid and invalid inputs.

Out of scope:

• Full Markdown parsing beyond fenced code blocks.
• Changing named-block or quoted-metadata syntax from issue Add stable block identities and robust metadata syntax #16.
• Reformatting fences or normalizing a users chosen fence style.

Verification status:
Confirmed: the parser and metadata-update scan currently accept only three- or four-backtick fences.