Documentation polish: coordinate guidance, module overview, minor doc warnings

Small documentation fixes, grouped so they can land as one PR.

Coordinate guidance (crate README)

• The Coordinates paragraph explains when rect needs shellac::transform::rect_page_space_to_user, but the same guidance applies to quad_points and user_point when they originate from a display frame. Say so, so all three fields are covered.
• The module overview (crate README, "what's public" paragraph) lists ops and transform but not annots. The top-level README already lists all three; align the crate README, since that is the copy rendered on crates.io.

rustdoc warnings

• annots.rs:5 has a bare URL (rustdoc::bare_urls, the only remaining cargo doc warning). Wrap it in <...>.

Comment wording

• report.rs: the C8 legend string hardcodes 10 for the loop iteration count (the value is pinned by a rendering test, so this is cosmetic; either derive it or leave a pointer to the constant).
• transform.rs:34,36,142,216: "still useful" / "remain" / "kept" / "remains" — tense words that imply a history the reader cannot see. Restate as present facts.
• min_max_x / min_max_y comments justify avoiding iterator chains via the Option return of min_by / max_by; the claim is broader than the cited reason. Narrow the wording.

Start with the crate README and compare its module overview with the top-level README, then inspect annots.rs, report.rs, and the cited locations in transform.rs. Run cargo doc to confirm the bare-URL warning is gone and run the existing rendering tests. Done means the coordinate guidance, module list, comments, and C8 legend wording are updated without changing behavior.