Rux script tier, milestone M8.
Wraps a rhai engine that holds the app's live state (the script's top-level
let variables persist in a Scope) and evaluates {{ }} bindings,
r-if/r-for expressions, and @tap handlers against it. Native
capabilities are exposed under the host:: namespace via the builder.
This replaces the M5 signal reader and the M6 inline-expression evaluator
with a real scripting language: named fn handlers, full expressions, and
the compiled-Rust boundary (docs/04-architecture.md, script/host tiers).