fmt deletes all comments

[danwt]

specl fmt strips every comment from a spec. Format-on-save in the VSCode extension means a user loses all comments on first save.

Repro (v3.1.1)

$ grep -c '//' vault.specl     # 20
$ specl fmt vault.specl | grep -c '//'   # 0

Any spec with comments reproduces it. fmt --write and the extension's format-on-save have the same effect on disk.

Cause

Comments are lexed as trivia (specl-syntax/src/token.rs:438, is_trivia) and the AST-based formatter never reattaches them. No test asserts comment preservation.

Impact

The README and the expert-specl skill both recommend fmt --write, and the extension does format-on-save. With comment-stripping that is destructive. Worth fixing before recommending format-on-save anywhere.

Suggested fix

Preserve comment tokens (attach to adjacent AST nodes as leading/trailing trivia) and re-emit them in the pretty-printer, plus a round-trip test that a spec with comments is unchanged by fmt --check.

[danwt]

Confirmed still present on main @8db9ede (built from source, stable 1.96.0, and run).

$ specl --version
specl 3.1.1  commit: 8db9ede  date: 2026-03-20
$ grep -c '//' vault.specl        # 20
$ specl fmt vault.specl | grep -c '//'   # 0

Cause unchanged: parser drops trivia before building the AST (crates/specl-syntax/src/parser.rs:50, .filter(|t| !t.kind.is_trivia()), where is_trivia covers Comment/DocComment). The AST has no comment field and crates/specl-syntax/src/pretty.rs never references Comment, so comments can't be re-emitted. The roundtrip tests added in 0c0cf23 compare parsed AST, which never contained the comments, so they don't catch this.