fix: preserve Mermaid edge curves on drag and warn on unbalanced artifact HTML

[auspham]

Summary

Fixes to the review surface, found while reviewing real plan/design artifacts.

1. Mermaid rearrange geometry (client/diagram.js). Several related defects in how a diagram renders and rebuilds, all visible once a diagram has parallel edges, curved edges, or a dense hub of labelled edges:

• Curves collapsed. Dragging a node redrew each connected edge as a straight line, flattening curves and stacking parallel edges between the same two nodes onto one identical segment. The rebuild now maps each edge's original path onto the moved endpoints with a similarity transform, so curves stay curved and parallel edges keep their separation.
• Arrowheads stacked. Endpoints were re-derived from node centres, so the fanned starts and arrowheads of parallel edges collapsed onto a single toward-centre point. Each endpoint is now pinned to its node's original attach point and moved by that node's drag delta.
• Bending jumped the arrowhead. Dragging a label to bend its edge used the same collapsed endpoints, so the arrowhead jumped to the shared point. The bend branch now uses the pinned endpoints and bows only the middle toward the cursor.
• Labels bound to the wrong edge. The edge-label to edge match compared positions in mismatched coordinate frames, assigned greedily in DOM order, and measured to each edge's arc-midpoint (which on a curved edge sits nowhere near the label) - so a label could bind to the wrong edge and a node drag then flung it across the diagram onto an unrelated arrow. Both positions are now lifted into the common frame and each label is paired to the edge it lies closest to (minimum distance along the whole path), globally-closest-first.
• Labels sat off their edge until dragged. Mermaid drops a label a little off a curved edge, and htmlit only snapped it on when the edge was dragged. Labels are now snapped onto their edge on a fresh render (a saved arrangement still wins), so they sit on their arrow from the start.

2. Warn the agent when artifact HTML has unbalanced block tags (htmlit_core/validation.py). htmlit serves the artifact verbatim, so a stray or unclosed <div>/<section> collapses the layout (a container closes early and following sections escape the centring wrapper, rendering full-width). The browser silently repairs the DOM, so the client cannot see it. A stdlib structure check re-reads the raw file whenever the client reports layout and folds any imbalance into layout_warnings as an unbalanced-html warning, naming each stray/unclosed tag with its line. It tracks only containers that require an explicit end tag, so it never false-positives on optional-end tags (<p>, <li>, <td>). No silent auto-repair: htmlit reports the problem so the fix lands in the author's source.

3. Make the answer/anchor jump two-way (client/annotate.js). Clicking a comment anchor already jumped to its agent answer, but there was no way back. A plain click on an answer block's "You asked:" header now scrolls to and flashes the anchored spot. The click is scoped to the header/block itself, not the answer body, so the answer text stays selectable and commentable; the header gets a pointer cursor and hover underline.

Type of change

• Bug fix
• New feature
• Documentation
• Refactor or tooling
• Other:

How was this tested?

Every change is TDD: each new test fails before its fix and passes after (verified by reverting each change and re-running its test).

• python -m pytest tests/python (45 passed, incl. new test_validation.py + server integration tests)
• ruff check skills/htmlit tests/python (clean)
• npm test (browser client, 19 passed, incl. parallel-edge curve/arrowhead, label-bend pinning, dense-diagram label matching across straight and curved edges, labels-on-line-at-load, answer-anchor jump, and the existing subgraph tests)

Checklist

• I added or updated tests for this change (a bug fix has a test that fails before the fix).
• I did not add any runtime dependency to the skill (standard library only).
• I did not commit vendored assets (Mermaid, highlight.js, Idiomorph).
• Commits follow Conventional Commits and are signed off (git commit -s).