Show bond hover previews in the browser editor

[dhsohn]

Motivation

I found that selecting Bond and moving the pointer over the browser canvas produced no faint preview until a mouse press. The unpressed pointer path refreshed only the Ring and Mark previews.

Changes

• Added a read-only bond-hover query using the shared drawing geometry, with a separate faint SVG overlay and target rings.
• Coalesced pointer requests and suppressed stale replies after movement, leave, press, tool/style changes or document changes. Hover leaves the accepted drawing, undo history and recovery draft unchanged.
• Added frontend interaction regressions and backend checks, including all 21 free/atom/bond × seven-style comparisons against the native two-carbon fixture. Updated the changelog.

Verification

• bash scripts/check.sh — passed on macOS with Python 3.13.14: 16,914 passed, 424 skipped and 1,036 passing subtests; Ruff, formatting and mypy passed. Full line coverage 94.40%, branch coverage 86.52%. RDKit was not installed.
• node --test --experimental-test-coverage tests/web_adapter.test.mjs — 187 passed. Against the merged baseline, the four new regressions fail while the existing 183 tests pass.
• Twenty-one actual HTTP preview requests preserved the accepted drawing response and recovery draft byte for byte. Served HTML and JavaScript matched the tested source.
• Visible browser interaction was not verified in this environment. The retained SVG was materialized from the HTTP response; it is not a browser screenshot.

Related issue

User-reported missing Bond hover preview in the browser editor.

Checklist

• The whole local gate passes, including file-isolated tests and contract checks.
• Added regression tests for the change.
• Architecture boundary tests pass.
• Updated CHANGELOG.md under Unreleased.
• Existing controls and usage are unchanged; no README update is needed.

Notes for reviewers

The preview uses the shared committed-drawing planner. Exact native geometry parity for labelled atoms, ring/neighbour context and perspective is not established by the two-carbon fixture. After a cancelled press, the preview resumes on the next pointer movement. Preview requests replan on a copy with one request in flight.