Add Unpacker and Prediffer fields to file filter expressions

[sdottaka]

This PR adds support for two new filter fields:

• Unpacker
• Prediffer

These fields allow users to filter files based on the plugin used during comparison.

Examples:

• Unpacker = "CompareMSExcelFiles"
• Prediffer != ""

Both fields are global attributes and do not support side-specific prefixes.

Changes include:

• Filter expression evaluation support
• UI integration in the filter helper menu
• Updates to the filter condition dialog
• Documentation updates

Notes:

• These fields return an empty string when no plugin is used
• Directories are excluded from evaluation

[Copilot]

Pull request overview

Adds two new global filter-expression fields—Unpacker and Prediffer—to allow filtering files by the plugin pipeline used during comparisons, with corresponding UI and documentation updates.

Changes:

• Extend filter expression evaluation to expose Unpacker/Prediffer as string fields.
• Integrate the new fields into the filter helper menu and filter condition dialog operator/UI logic.
• Update manual/documentation strings (including translation template updates) to describe the new fields and usage examples.

Reviewed changes

Copilot reviewed 10 out of 15 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
Src/FilterEngine/FilterExpressionNodes.cpp	Adds Unpacker/Prediffer field evaluators and wires them into field-name parsing.
Src/FileFilterHelperMenu.cpp	Adds helper-menu actions to build expressions using Unpacker/Prediffer.
Src/FilterConditionDlg.cpp	Updates operator/“Match case” behavior by classifying Unpacker/Prediffer as string fields.
Src/FilterConditionDlg.h	Declares the new IsStringField() helper used by the dialog logic.
Src/resource.h	Introduces command IDs for the new helper-menu actions and updates menu ID ranges.
Translations/Docs/Manual/English.pot	Updates the translation template with new manual entries for Unpacker/Prediffer and reference shifts.

Comments suppressed due to low confidence (1)

Src/FilterEngine/FilterExpressionNodes.cpp:1134

• Global fields (side == -2) still accept Left/Middle/Right prefixes because the prefix handling happens before the side == -2 check. For example, an expression like LeftUnpacker will be parsed and then side < 0 math maps it to a real index (and in 3‑way it even maps to the wrong side). Since the docs say Unpacker/Prediffer are global and must not support side-specific prefixes, reject prefixed variants (e.g., throw invalid field when prefixlen>0 && side==-2) instead of evaluating them.

	else if (strcmp(p, "unpacker") == 0)
	{
		functmp = UnpackerField;
		side = -2;
	}
	else if (strcmp(p, "prediffer") == 0)
	{
		functmp = PredifferField;
		side = -2;
	}
	else
		throw std::runtime_error("Invalid field name: " + std::string(v.begin(), v.end()));
	if (prefixlen > 0)
		func = [side, functmp](const FilterExpression* ctxt, const DIFFITEM& di)-> ValueType { return functmp(side < 0 ? ctxt->ctxt->GetCompareDirs() + side: side, ctxt, di); };
	else
	{
		if (side == -2)
		{
			func = [functmp](const FilterExpression* ctxt, const DIFFITEM& di)-> ValueType { return functmp(-2, ctxt, di); };

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