feat(reflow): add atomic_spans configuration and refactor inline wrapping

[chandlerc]

Introduce atomic_spans configuration (defaulting to true) to MD013
to control whether inline formatting spans (emphasis, strong, strikethrough,
and code spans) are treated as atomic units or allowed to wrap at whitespace.

Refactor wrapping and sentence-splitting logic with extensive optimizations:

• Add safety guards in eligibility checks to prevent wrapping inside spans
  that contain nested format markers (*, _, ~, ```), preventing syntax
  corruption.
• Prevent wrapping from starting a line with definition lists (:), Pandoc
  divs (:::), or table rows (|) to avoid accidental block construct triggers.
• Optimize sentence boundary checks from O(N^2) to O(1) by passing pre-computed
  byte offsets, resolving performance scaling bottlenecks.
• Eliminate paragraph-sized Vec<bool> mask allocations for inline code spans by
  maintaining sorted code span indexes inline.
• Implement allocation-free Element::display_len to avoid string formatting
  just to measure visual lengths, and defer format!("{element}") calls to the
  final emission.
• Optimize config budget calculation in min_effective_line_length using stack-
  allocated arrays instead of vectors.
• Single-pass character collection in the sentence splitter.

Reverts the three emphasis_spans commits that this replaces.

Assisted-by: Antigravity with Gemini