fix: render session watermark on scratch terminal

[umputun]

Summary

• let scratch surfaces inherit the owning session's visual config without wiring their OSC title or PWD into the session model
• apply live watermark set and clear operations to realized scratch surfaces and reassert them across config and opacity reloads
• update the control and libghostty lifecycle notes

Verification

• swift test: 1,727 tests passed
• swiftlint lint --strict --quiet
• Debug xcodebuild succeeded
• demonstrated in an isolated dev instance with the watermark visible on the scratch terminal

Closes #274

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