Refactor attach_orphan_specifiers to use state machine with Peekable iterator

[coderabbitai]

Problem

The current implementation of attach_orphan_specifiers in src/fences.rs uses a complex reverse-search and removal pattern that makes the code harder to follow:

• Uses .rev().find(…) to search backwards through the output buffer
• Performs out.remove(idx) operations to splice out orphaned specifiers
• Complex index management logic

Suggested Solution

As suggested by sourcery-ai[bot], rewrite the function as a state machine using a Peekable iterator. This approach would:

• Eliminate the reverse-search + out.remove(idx) back-splice
• Use one pass with peekable() to match "lang" + "```" pairs
• Keep all existing behaviour and tests intact
• Be much easier to follow and maintain

The suggested implementation uses a forward-looking approach that detects orphaned language specifiers by peeking ahead to see if the next line is exactly "```".

Context

• Original discussion: Add fences option #100 (comment)
• PR: Add fences option #100
• File: src/fences.rs
• Function: attach_orphan_specifiers

Acceptance Criteria

• Rewrite attach_orphan_specifiers using a Peekable iterator approach
• Ensure all existing tests continue to pass
• Verify the same behaviour is maintained
• Code should be easier to read and understand