Reuse placeholder nodes on each thread to reduce allocation #5303

Job	Run time
Check	1m 10s
Clippy	1m 26s
Rustfmt	22s
matrix_test (x86_64-pc-windows-msvc, windows-latest, 1.75.0, false, test, --all --tests)	10m 12s
Playwright Tests	7m 56s
Test Suite	5m 34s
matrix_test (x86_64-apple-darwin, macos-latest, 1.75.0, false, test, --all --tests)	5m 4s
matrix_test (aarch64-apple-ios, macos-latest, 1.75.0, false, build, --package dioxus-mobile)	1m 12s
matrix_test (aarch64-linux-android, ubuntu-latest, 1.75.0, true, build, --package dioxus-mobile)	1m 33s
	34m 29s

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