Allocate 16k scratch on heap #831

Job	Run time
sanity-test-run	3m 41s
alpine-linux-x86 (/awslc/tests/ci/run_fips_tests.sh, --build-arg CC=clang --build-arg CXX=clang++)	29m 55s
clang-18-pedantic	0s
clang-ubuntu-2004-sanity	0s
alpine-linux-x86 (/awslc/tests/ci/run_fips_tests.sh, --build-arg CC=gcc --build-arg CXX=g++)	27m 54s
clang-ubuntu-2204-sanity	0s
alpine-linux-x86 (/awslc/tests/ci/run_posix_tests.sh, --build-arg CC=clang --build-arg CXX=clang++)	1h 6m 1s
alpine-linux-x86 (/awslc/tests/ci/run_posix_tests.sh, --build-arg CC=gcc --build-arg CXX=g++)	1h 4m 15s
clang-ubuntu-2404-sanity	0s
FreeBSD ${{ matrix.version }} test	0s
gcc-13-pedantic	0s
gcc-4_8	6m 46s
gcc-ubuntu-2004-sanity	0s
gcc-ubuntu-2204-sanity	0s
gcc-ubuntu-2404-sanity	0s
macOS-ARM	0s
macOS-ARM-FIPS	0s
macOS-x86	0s
macOS-x86-FIPS	0s
MSVC-2019	0s
MSVC-2022	0s
MSVC-SDE-64-bit	0s
OpenBSD-x86-64 (7.4)	45m 34s
OpenBSD-x86-64 (7.5)	47m 38s
OpenBSD-x86-64 (-f, 7.4)	47m 32s
OpenBSD-x86-64 (-f, 7.5)	44m 39s
	6h 23m 55s

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