fix(looks-same): expose native diff percentage

[Octember]

Summary

• preserve the existing lightweight Looks-Same boolean comparison path
• when VRT already requests a saved diff, use Looks-Same native createDiffImage output
• persist differentPixels / totalPixels as diffPercent alongside that artifact

Why

Looks-Same already calculates differentPixels and totalPixels while producing a diff image. VRT currently discards those metrics, so unresolved comparisons display Diff: 0%. The metrics stay strictly in the existing saveDiffAsFile path: ordinary comparisons do not allocate a diff image or change behavior.

Verification

• real Looks-Same diff test asserts image artifact, mismatch count, and 50% result
• npm test -- --runInBand src/compare/libs/looks-same/looks-same.service.spec.ts
• npm run build
• focused ESLint and Prettier checks

Summary by CodeRabbit

• Bug Fixes
  • Diff results now include the diff name, percentage difference, and mismatched pixel count.
  • Diff images are saved consistently when requested.
  • Comparing identical images now reports an explicit error instead of silently omitting the diff.

[pashidlos]

Bugbot found 1 issue on PR #354 (Bugbot).

Severity	Location (file:line)	Finding
high	src/compare/libs/looks-same/looks-same.service.ts:87	createDiff no longer swallows looksSame errors and throws on equal: true. calculateDiff deletes the old diff file before getDiff, so a throw skips saveDiffResult and can leave a stale diffName pointing at a removed file.

[coderabbitai]

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