fix: restore highlights for cross-node and cross-paragraph selections

[kasunben]

Summary

• Root cause: serializeRange only stored xpath (start container path); for any selection where startContainer ≠ endContainer, restoreRange applied endOffset to the wrong node — producing a null range or a truncated highlight on every page reload
• Fix 1 — anchor shape: added endXpath: _getXPath(range.endContainer) to the serialized anchor; restoreRange now resolves start and end nodes independently with anchor.endXpath || anchor.xpath fallback for backward compat with existing IDB/server data
• Fix 2 — multi-mark highlight: range.surroundContents() throws HierarchyRequestError for cross-element ranges; highlightRange now catches this and falls back to _highlightRangeMulti, which collects all text nodes via NodeIterator before any DOM mutation (prevents iterator invalidation), then wraps each in its own <mark>
• _allMarksOf(mark): new helper returning mark._allMarks || [mark]; all downstream operations (unwrap, is-resolved class, click listeners, _threadId assignment) iterate the full mark group atomically
• Bumped version to 0.5.14; rebuilt annotate.min.js

Type

• Bug fix

Affected modes

All modes (offline, BroadcastChannel, server-sync, P2P) — anchor serialization and highlight rendering are shared across all sync modes.

Testing

• Cross-paragraph selection → highlight covers both paragraphs; reload → both segments re-highlighted; card positioned at start of selection
• Inline-element selection (e.g. text spanning <strong>) → multi-segment highlight; reload → fully restored
• Single-node selection (same text node, start = end) → fast path unchanged; existing behaviour preserved
• Legacy anchors (no endXpath in IDB/server) → restoreRange falls back to anchor.xpath for both nodes; no regressions
• Resolve / un-resolve on multi-mark highlight → is-resolved class applied/removed on all marks atomically
• Unwrap (delete) on multi-mark highlight → all mark segments removed cleanly

Breaking changes

None. The endXpath field is additive; restoreRange is fully backward compatible with anchors that lack it.

Checklist

• Code change in assets/js/annotate.js
• annotate.min.js rebuilt (npm run build)
• Version bumped (0.5.13 → 0.5.14)
• README.md updated (anchor data model, core checklist, Roadmap Shipped)
• CLAUDE.md updated (anchor shape, Highlight Re-anchoring section, Phase M, Key Implementation Notes)

Closes #29

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