Complete implementation of Git-style change metadata and diff utilities

- Finalized the `ChangeMetadata` module to compute structured change metadata, replacing inline calculations in `PatchTree`.
- Introduced `DiffUtil` for accurate line difference calculations using the Myers algorithm, ensuring consistency with Git's behavior.
- Updated `DomLabel` to display git-style change indicators, including added and removed line counts, with hover tooltips for detailed information.
- Comprehensive tests for `DiffUtil` have been added to validate functionality across various scenarios.
- Removed the obsolete `WhitespaceUtil` as its functionality is now integrated into the new modules.

plugin/src/ChangeMetadata.lua

@@ -0,0 +1,159 @@
+--[[
+	Computes structured change metadata from a changeList.
+
+	Produces both summary fields (for display tags and diff bar) and
+	detail fields (for future merge dialogue). Replaces the ad-hoc
+	inline metadata computation that was previously in PatchTree.build().
+]]
+
+local Rojo = script:FindFirstAncestor("Rojo")
+local Plugin = Rojo.Plugin
+
+local DiffUtil = require(Plugin.DiffUtil)
+
+local ChangeMetadata = {}
+
+--[[
+	Compute metadata for an updated instance's changeList.
+	changeList is an array of {prop, currentValue, incomingValue} entries
+	(without the header row -- that is added separately by PatchTree).
+
+	Returns a ChangeMetadata table with:
+	  Summary (for display):
+	    linesAdded, linesRemoved, isWhitespaceOnly, propChanges
+	  Detail (for future merge dialogue):
+	    hunks, propertyDetails
+	  Tracking:
+	    edits, failed (set later by updateMetadata)
+]]
+function ChangeMetadata.compute(changeList: { { any } }): { [string]: any }
+	local linesAdded: number? = nil
+	local linesRemoved: number? = nil
+	local isWhitespaceOnly: boolean? = nil
+	local hunks: { any }? = nil
+	local propertyDetails: { any }? = nil
+	local propCount = 0
+	local totalEdits = #changeList
+
+	for _, entry in changeList do
+		local prop = entry[1]
+		local currentValue = entry[2]
+		local incomingValue = entry[3]
+
+		if prop == "Source" and type(currentValue) == "string" and type(incomingValue) == "string" then
+			-- Source property: compute line diff (always exact, no thresholds)
+			local diffResult = DiffUtil.diffLines(currentValue, incomingValue)
+
+			if diffResult.added > 0 or diffResult.removed > 0 then
+				linesAdded = diffResult.added
+				linesRemoved = diffResult.removed
+				isWhitespaceOnly = diffResult.isWhitespaceOnly
+				hunks = diffResult.hunks
+			end
+		else
+			-- Non-Source property
+			propCount += 1
+
+			-- Build property detail for future merge dialogue
+			if not propertyDetails then
+				propertyDetails = {}
+			end
+
+			table.insert(propertyDetails, {
+				name = prop,
+				oldValue = currentValue,
+				newValue = incomingValue,
+				valueType = typeof(incomingValue) or typeof(currentValue) or "unknown",
+			})
+		end
+	end
+
+	return {
+		-- Summary (for display tags and diff bar)
+		linesAdded = linesAdded,
+		linesRemoved = linesRemoved,
+		isWhitespaceOnly = isWhitespaceOnly,
+		propChanges = if propCount > 0 then propCount else nil,
+
+		-- Tracking (edits used by added instances and failure tracking)
+		edits = totalEdits,
+
+		-- Detail (for future merge dialogue -- not consumed by UI yet)
+		hunks = hunks,
+		propertyDetails = propertyDetails,
+	}
+end
+
+--[[
+	Compute metadata for an added instance's changeList.
+	These are all new properties, so there are no "current" values.
+	changeList entries are {prop, "N/A", incomingValue}.
+]]
+function ChangeMetadata.computeForAddition(changeList: { { any } }): { [string]: any }
+	local linesAdded: number? = nil
+	local hunks: { any }? = nil
+	local propertyDetails: { any }? = nil
+
+	for _, entry in changeList do
+		local prop = entry[1]
+		local incomingValue = entry[3]
+
+		if prop == "Source" and type(incomingValue) == "string" then
+			-- New script: count lines being added
+			local diffResult = DiffUtil.diffLines("", incomingValue)
+			if diffResult.added > 0 then
+				linesAdded = diffResult.added
+				hunks = diffResult.hunks
+			end
+		end
+
+		if not propertyDetails then
+			propertyDetails = {}
+		end
+
+		table.insert(propertyDetails, {
+			name = prop,
+			oldValue = nil,
+			newValue = incomingValue,
+			valueType = typeof(incomingValue) or "unknown",
+		})
+	end
+
+	return {
+		edits = #changeList,
+		linesAdded = if linesAdded then linesAdded else nil,
+		linesRemoved = if linesAdded then 0 else nil,
+		isWhitespaceOnly = false,
+		hunks = hunks,
+		propertyDetails = propertyDetails,
+	}
+end
+
+--[[
+	Compute metadata for a removed instance.
+	Reads the Source property (if it exists) to show line counts.
+	Returns nil if the instance has no Source (non-script removal).
+]]
+function ChangeMetadata.computeForRemoval(instance: Instance): { [string]: any }?
+	local ok, source = pcall(function()
+		return (instance :: any).Source
+	end)
+
+	if not ok or type(source) ~= "string" then
+		return nil
+	end
+
+	local diffResult = DiffUtil.diffLines(source, "")
+
+	if diffResult.removed == 0 then
+		return nil
+	end
+
+	return {
+		linesAdded = 0,
+		linesRemoved = diffResult.removed,
+		isWhitespaceOnly = false,
+	}
+end
+
+return ChangeMetadata