fix: capture valid Unity editor screenshots

[Daliys]

Summary

• Replace macOS screencapture calls for Game View and Inspector with Unity-native InternalEditorUtility.ReadScreenPixel PNG encoding.
• Use point-space editor bounds, repaint/frame retry handling, structured screenshot metadata, and preserve legacy response fields.
• Add EditMode coverage, 20-capture smoke validation, documentation updates, and project-memory records.

Validation

• Live Unity 6000.4.3f1 smoke: Game View 20/20 valid PNG captures; Inspector 20/20 valid PNG captures.
• Static pre-push validation: 43 Python bridge tests passed; quality gate 0 errors and 1 advisory warning.
• Pre-push quick live smoke passed: 117 raw tools, server Running.
• Unity editor compile/idle state clean.

Notes

• The Assets-based parent harness does not discover the package Tests~ EditMode assembly, so those tests are present but could not be executed by that harness.
• Existing lint findings are unrelated file-size and broken-prefab findings.