Prepare 0.3.0 release and split Linux/macOS distribution paths

[OneNoted]

Summary

• add the shared Rust core and native macOS preview host scaffolding
• split public distribution so Linux uses the taskers launcher while macOS ships from GitHub Releases DMGs
• harden Linux desktop-entry registration so repo-local launcher runs cannot poison user desktop entries
• bump the workspace to 0.3.0 and update the release checklist for the new publish order and Cargo dry-run behavior

Validation

• cargo test
• cargo check --workspace
• bash scripts/smoke_taskers_ui.sh
• bash scripts/smoke_taskers_focus_churn.sh
• bash scripts/build_linux_bundle.sh
• bash scripts/smoke_linux_release_launcher.sh
• cargo publish --dry-run -p taskers-domain
• cargo publish --dry-run -p taskers-paths
• cargo install --offline --path crates/taskers-app

Notes

• macOS signing, notarization, DMG verification, and AppKit smoke validation still need the hosted macOS workflows; they were not run from this Linux machine
• publishing is intentionally not part of this PR; this is the review branch before tag and release work

[OneNoted]

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Guard launcher target triple against musl builds

current_target_triple() hardcodes every x86_64 Linux build to x86_64-unknown-linux-gnu, while the compile-time gate only checks target_os/target_arch. That means cargo install taskers still succeeds on x86_64-unknown-linux-musl systems (e.g., Alpine), but the launcher then downloads the glibc bundle and fails when executing bin/taskers due to missing glibc loader support. Please reject non-gnu targets (for example with target_env = "gnu") or resolve artifacts from the full target triple.

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