Explain movement-masked stress hours

[kavemang]

Summary

• show the localized activity-masked hour count on the Stress timeline and hosted Today stress card on Android and Apple
• replace Android's partly hardcoded timeline caption with complete day-relative and personal-baseline resources
• describe the daytime signal without claiming that the currently disabled daytime HRV input is active
• carry Android's canonical non-overlapping masked-hour count into the Today card so half-hour display windows cannot inflate it

Verification

• cd android && ./gradlew :app:compileFullDebugKotlin
• cd android && ./gradlew :app:testFullDebugUnitTest --tests 'com.noop.analytics.DaytimeStressTest' --tests 'com.noop.widget.StressTraceTest' --rerun-tasks
• python3 Tools/i18n_audit.py --ci upstream/main
• python3 Tools/doc_comment_lint.py
• JSON and Android XML resource validation

Apple app targets were not compiled locally because this development host is Linux; the app-target changes are covered by upstream Apple CI.

Closes #2125