Reorganize Fission facade feature boundaries

[zcourts]

Summary

• Keeps the fission facade as the single dependency for applications while making platform shells and heavier integrations opt-in.
• Adds facade features for desktop, web, mobile targets, static sites, charts, 3D, terminal widgets, icon reflection, and test driver APIs.
• Moves terminal widget dependencies behind terminal-widget and moves 3D shell rendering behind three-d.
• Updates examples, CLI templates, and docs/reference snippets to use the facade instead of direct internal crate dependencies.
• Fixes stale semantics test initializers for the current action_scope_id field.

Verification

• cargo test --workspace
• cargo check -p fission --no-default-features
• cargo check -p fission --features desktop,three-d,terminal-widget
• cargo check -p fission-shell-web --target wasm32-unknown-unknown
• cargo check -p fission-shell-mobile --target aarch64-apple-ios
• cargo run -p fission-cli --bin fission -- site check --project-dir documentation --release
• Dependency checks confirm default/site/web/mobile paths do not pull terminal deps or fission-3d unless the matching feature is enabled.

Notes

Existing warnings remain in render/editor/example code; no workspace test failures remain.