fix: preserve walrus comments across formatter passes

[msaule]

Fixes #512.

Summary

Preserve an own-line comment between := and its RHS so that formatting is idempotent. Comment-free walrus line breaks and end-of-line comments retain Air's existing compact canonical output.

Root cause

Walrus formatting intentionally flattens ordinary line breaks. When the RHS instead starts with an own-line comment, flattening changes the comment's syntactic position; a second pass then moves it after the RHS.

Validation

• cargo test -p air_r_formatter
• cargo test --workspace --exclude lsp
• cargo clippy -p air_r_formatter --all-targets
• compiled CLI two-pass checks for own-line, comment-free, and end-of-line comment cases