BridgeJS: Unify optional stack encoding to presence-flag form #2759

Job	Run time
Build for native target (macos-15, Xcode_26.0.1)	3m 17s
Build and Test (ubuntu-24.04, https://download.swift.org/development/ubuntu2404/swift-DEVELOPMENT...	16m 1s
build-examples (PlayBridgeJS)	12m 18s
check-bridgejs-generated	3m 22s
format	37s
build-examples (Basic)	10m 24s
prettier	12s
build-examples (ActorOnWebWorker)	10m 0s
build-examples (Embedded)	10m 23s
build-examples (OffscrenCanvas)	8m 51s
Build and Test (ubuntu-22.04, https://download.swift.org/development/ubuntu2204/swift-DEVELOPMENT...	15m 32s
Build and Test (ubuntu-24.04, https://download.swift.org/swift-6.3-branch/ubuntu2404/swift-6.3-DE...	13m 55s
build-examples (Multithreading)	10m 53s
Test BridgeJS against Swift versions (swift:6.3, 603.0.0)	3m 14s
build-examples-deploy	-1s
deploy-examples	-1s
	1h 58m 57s

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