Port the pen-ai-skills diagnostics layer to a new pure Rust crate
`op-design-lint`: 14 design-lint detectors, the detect_all aggregator,
apply_fixes / detect_and_fix, and golden parity tests against the TS
oracle. Wire it into op-mcp as the read-only debug_validation_report
tool, gated by OPENPENCIL_DEBUG_TOOLS=1.
Detectors: empty_paths, unexpected_rotation, excessive_frame_effects,
invisible_containers, text_explicit_heights, text_effect,
text_corner_radius, text_stroke, text_bg_contrast, edge_section_padding,
stacked_horizontal_padding, sibling_inconsistencies (+ check_consistency),
detect_all.
Also includes: node_util shared helpers + pen-core color/visibility
ports, node_mut field accessors, set_property issue->node mutation
dispatch, golden fixture corpus + TS dump script, structural-parity
test, a CI golden-drift guard, and the gitignore fix so the fixture
docs/ dir is tracked.
This branch's per-commit history was squashed: the original 28 commits
carried fabricated timestamps and could not be honestly reconstructed,
so the work is recorded as a single commit at its real completion time.
8cff5abaf5