Add LiveView HEEx lowering

[dannote]

Summary

• add a LiveView plugin with HEEx source support and ~H AST lowering
• introduce source-origin metadata for template labels and spans
• route plugin source extensions through project/file parsing and hide LiveView rendering helper edges
• add LiveView semantic edges for assigns, streams, JS.push/push_event handlers, and parser-lowered component attrs

Notes

• For LiveView 1.2 parser-backed templates, Reach lowers HEEx through a Reach-owned HEEx IR so :if, :for, static phx-* event attrs, and component calls get cleaner source labels and control/data-flow.
• For current LiveView 1.1.x projects, Reach falls back through TagEngine; assign flow, stream flow, and JS.push/push_event event wiring are active, while component attr flow avoids generated runtime helper calls to prevent noisy edges.
• Manually reviewed semantic edge output from the external smoke run and tightened component attr edges after finding generated slot/runtime noise.

Validation

• MIX_ENV=test mix compile --warnings-as-errors
• MIX_ENV=test mix test test/reach/frontend/elixir_test.exs test/reach/project/project_test.exs test/reach/reach_test.exs test/reach/plugin/lower_elixir_ast_test.exs test/reach/plugins/live_view_test.exs test/reach/plugins/live_view/heex_lowerer_test.exs test/reach/plugins/live_view/semantics_test.exs test/reach/visualize/control_flow/block_quality_test.exs
• external smoke against 10 LiveView projects: phoenix_live_view, phoenix_live_dashboard, livebook, plausible/analytics, ash_hq, firezone, blockscout, keila, live_beats, surface