BridgeJS: Unify optional stack encoding to presence-flag form #2758

Job	Run time
Build for native target (macos-15, Xcode_26.0.1)	2m 16s
format	36s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	9m 29s
build-examples (PlayBridgeJS)	9m 29s
Test BridgeJS against Swift versions (swift:6.3, 603.0.0)	3m 21s
Build and Test (ubuntu-24.04, https://download.swift.org/swift-6.3-branch/ubuntu2404/swift-6.3-DE...	9m 29s
build-examples (OffscrenCanvas)	9m 28s
build-examples (Embedded)	9m 28s
build-examples (Multithreading)	9m 28s
build-examples (Basic)	9m 26s
build-examples (ActorOnWebWorker)	9m 29s
prettier	12s
Build and Test (ubuntu-24.04, https://download.swift.org/development/ubuntu2404/swift-DEVELOPMENT...	9m 29s
check-bridgejs-generated	3m 40s
deploy-examples	0s
build-examples-deploy	-10s
	1h 35m 10s

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