chore(comfort): Add semantic line-break formatter for doc comments

[JeanMertz]

Introduce comfort, a new contrib tool that reformats Rust doc-comment blocks (/// and //!) using semantic line breaks — one sentence per line — with an optional --max-width safety net.

The tool ships as two binaries that share a single entry-point: comfort (invoked directly) and cargo-comfort (a cargo subcommand). Direct invocation without paths defaults to stdin/stdout; cargo comfort defaults to walking the current workspace.

The pipeline runs in three layers:

• extract: tokenises Rust source via ra-ap-rustc_lexer, finding maximal runs of consecutive /// / //! lines and collecting their indentation, style, and body text.
• format: parses each block's body with comrak, identifies top-level paragraph spans, and hands their text to the reflow step. All other markdown structure — reference link definitions, block quotes, lists, code blocks, headings — is preserved verbatim.
• sentence: splits paragraphs into individual sentences using UAX#29 (unicode-segmentation), with abbreviation-aware merging and inline-token protection (URLs, code spans, markdown links) via regex placeholders, then width-wraps each sentence with textwrap.

A fmt-comments-ci just task is added to the CI pipeline:

comfort --check --workspace

It exits non-zero and prints a unified diff for any file that would be reformatted, integrating with the existing CI matrix.