BridgeJS: Unify optional stack encoding to presence-flag form #141

Job	Run time
format	56s
Test BridgeJS against Swift versions (swift:6.3, 603.0.0)	3m 24s
Build and Test (ubuntu-24.04, https://download.swift.org/development/ubuntu2404/swift-DEVELOPMENT...	14m 39s
check-bridgejs-generated	3m 34s
Build and Test (ubuntu-24.04, https://download.swift.org/swift-6.3-branch/ubuntu2404/swift-6.3-DE...	13m 23s
Build for native target (macos-15, Xcode_26.0.1)	3m 12s
prettier	11s
build-examples (OffscrenCanvas)	10m 20s
build-examples (ActorOnWebWorker)	10m 45s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	20m 32s
build-examples (Embedded)	9m 28s
build-examples (Multithreading)	10m 7s
build-examples (Basic)	9m 59s
build-examples (PlayBridgeJS)	12m 11s
deploy-examples	0s
build-examples-deploy	0s
	2h 2m 41s

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