Release v0.2.4

[yvette-carlisle]

Summary

• release v0.2.4 in Cargo metadata, build fallback, and release docs
• keep Settings -> About -> Check available even when Sparkle reports an active update session
• add native-host probe coverage for manual update check availability

Verification

• cargo make fmt
• cargo make test-macos-native-host
• cargo make checks
• cargo make test-host-reset
• cargo make test-macos-native-host-stage
• scripts/smoke/sparkle-update-local.sh --self-check
• scripts/smoke/sparkle-update-local.sh --prepare-only
• scripts/smoke/macos.sh
• scripts/perf/macos.sh after rerunning one transient HUD event-delivery flake
• EXPECT_SCROLL_COPY_CACHE_HIT=1 SCROLL_START_METHOD=keyboard scripts/smoke/native-scroll-capture-macos.sh
• EXPECT_SCROLL_COPY_CACHE_HIT=1 SCROLL_START_METHOD=toolbar scripts/smoke/native-scroll-capture-macos.sh
• bash -n scripts/build_and_run.sh scripts/release/sparkle-appcast.sh scripts/smoke/native-scroll-capture-macos.sh scripts/smoke/sparkle-update-local.sh
• semantic drift audit reviewed as pass: update-check availability and v0.2.4 release claims map to executable policy/probe/version evidence

Manual Sparkle install/relaunch was not run in this PR; local Sparkle self-check and prepare-only packaging/appcast checks passed.