feat(allure): screen diff plugin support

[Raytek]

This PR aims to add support to allure screen diff plugin.

resolves #70
resolves #58

[Copilot]

Pull Request Overview

Adds Allure screen-diff plugin support by changing how screenshots are attached when a visual mismatch exceeds tolerance.

• Replace three image attachments with a single allure.addScreenDiff call.
• Switch from Buffer attachments to base64-encoded strings.

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[Copilot]

CodeceptJS's Allure plugin does not expose addScreenDiff in supported versions; calling this will throw at runtime. Use standard attachments with the names expected by the Allure screen-diff plugin (expected, actual, diff), or feature-detect and fall back to addAttachment.

Suggested change

	allure.addScreenDiff(
	"Screen Diff",
	fs.readFileSync(this._getBaseImagePath(baseImage, options), { encoding: "base64" }),
	fs.readFileSync(this._getActualImagePath(baseImage), { encoding: "base64" }),
	fs.readFileSync(this._getDiffImagePath(baseImage), { encoding: "base64" }),
	);
	allure.addAttachment(
	"expected",
	fs.readFileSync(this._getBaseImagePath(baseImage, options)),
	"image/png"
	);
	allure.addAttachment(
	"actual",
	fs.readFileSync(this._getActualImagePath(baseImage)),
	"image/png"
	);
	allure.addAttachment(
	"diff",
	fs.readFileSync(this._getDiffImagePath(baseImage)),
	"image/png"
	);

[Copilot]

Reading images as base64 strings increases memory/cpu and bloats attachments by ~33%. Prefer passing Buffers (omit the encoding option) to avoid unnecessary encoding and to keep report size smaller.

Suggested change

	fs.readFileSync(this._getBaseImagePath(baseImage, options), { encoding: "base64" }),
	fs.readFileSync(this._getActualImagePath(baseImage), { encoding: "base64" }),
	fs.readFileSync(this._getDiffImagePath(baseImage), { encoding: "base64" }),
	fs.readFileSync(this._getBaseImagePath(baseImage, options)),
	fs.readFileSync(this._getActualImagePath(baseImage)),
	fs.readFileSync(this._getDiffImagePath(baseImage)),