ci(cpp): widen memory_per_step_growth_kb threshold to 75% #1784

Job	Run time
discover-apps	47s
build-apps (windows-latest, jira, src/gaia/apps/jira/webui, GAIA JIRA Dashboard, 1.0.0)	2m 4s
build-apps (ubuntu-latest, jira, src/gaia/apps/jira/webui, GAIA JIRA Dashboard, 1.0.0)	1m 24s
build-apps (windows-latest, example, src/gaia/apps/example/webui, Example App - MCP Demo, 1.0.0)	2m 22s
build-apps (ubuntu-latest, example, src/gaia/apps/example/webui, Example App - MCP Demo, 1.0.0)	38s
	7m 15s

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