chore(lints): adopt surgical clippy opt-ins workspace-wide

[lostmygithubaccount]

Summary

• Enable six high-signal clippy lints at [workspace.lints.clippy] and fix every existing violation across all 6 crates (including draw-py, which is its own workspace and mirrors the block manually).
• Lints chosen: manual_let_else, missing_errors_doc, missing_panics_doc, redundant_closure_for_method_calls, uninlined_format_args, semicolon_if_nothing_returned.
• Skipped pedantic wholesale plus must_use_candidate and needless_pass_by_value — the f32/f64 cast warnings and getter-spam would overwhelm the real signals without catching any bugs in this codebase.

Notable refactors

• export_svg no longer splices <defs> in via svg.find('\n').unwrap() + 1. Assembles header → defs → body → footer in order.
• find_nearest_snap_point uses Option::is_none_or instead of is_none() || .unwrap().
• update_element_style (wasm) replaces three match { _ => return false } chains with let…else.
• .map(|x| x.method()) rewrites to fn-item refs where trait dispatch is unambiguous (String::as_str, Element::id, f64::to_string).
• # Errors / # Panics docs added on export_png, export_svg, the storage API, run_webapp, run_app, run_cli, and Renderer::render. Each names the actual failure surface rather than boilerplate.

Reviewed via /critique

Round 1: correctness + design + performance reviewers. Incorporated fixes for misleading # Panics text on Renderer::render, over-broad # Errors on run_cli / run_app, duplicate export_png docs, stale export_svg comment, and the draw-py coverage gap. Performance reviewer flagged a small extra allocation in the no-defs SVG path; kept for readability (sub-ms on realistic inputs) but documented as a possible future follow-up.

Test plan

• bin/check green locally (fmt + clippy -D warnings + cargo test --workspace + Python ruff/ty)
• Hachure-fill SVG output inspected for byte equivalence vs. pre-refactor
• CI green