Replace positional diff with CollectionDifference-based shape matching

[wojo]

Summary

Alternative fix for #482 — replaces the naive positional diff with Swift's CollectionDifference using shape-based identity matching. This correctly handles mid-list inserts and removes without misaligning item properties.

See #483 for the simpler fallback approach (Option 1).

How it works

Each MenuItemNode gets a ShapeFingerprint that captures its structural role:

• isSeparator — separators match other separators
• hasImage — image-only lines are distinct from text
• fontName — monospace lines (e.g. font=Menlo) are distinct
• titleKey — stable prefix up to first : or first word ("Battery:", "Climate:", "Home")
• hasChildren / hasFold — structural distinctions

An occurrence counter disambiguates duplicates (e.g. multiple separators). CollectionDifference.inferringMoves() uses these tagged nodes to correctly track items across position shifts.

Pros / Cons vs Option 1 (#483)

Option 1: Full rebuild on structural change (#483)

Pros:

• Minimal code change (~15 lines)
• Zero risk of new edge cases
• Easy to review and reason about

Cons:

• Loses live menu updates when items appear/disappear (menu flickers on rebuild)
• Fold expansion state lost on structural changes
• Webview state destroyed on structural changes
• Doesn't fulfill the original goal of Feature request: reuse menu items instead of recreating them #467 for the insert/remove case

Option 2: CollectionDifference shape matching (this PR)

Pros:

• Menu stays fully live during all updates — no flicker on insert/remove
• Fold expansion state preserved across structural changes
• Webview state preserved
• Fulfills the original intent of Feature request: reuse menu items instead of recreating them #467 for all cases
• Uses Swift's battle-tested LCS algorithm, not a custom diff

Cons:

• More code (~150 lines added)
• Shape fingerprint heuristics may need tuning for edge cases (e.g. two items with identical shapes)
• Falls back to remove+insert for true reordering (rare for plugins)

Files changed

• MenuItemNode.swift — Added ShapeFingerprint struct, shapeFingerprint computed property, extractTitleKey() helper
• MenuDiff.swift — Replaced diffMenuNodes() with CollectionDifference-based implementation, added TaggedNode wrapper

Test plan

• Repro script from Incremental update misaligns items around large images, rendering ghost text beside image #482 — no ghost text beside image with menu open
• Values-only updates still patch in-place (no rebuild)
• Insert/remove correctly tracks shifted items
• Fold sections preserve expansion state across structural changes
• Existing SwiftBar tests pass