v0.0.13: State<T> WASM host imports (C6d)

C6d: State<T> WASM Host Imports

Compile get/put operations for State<T> effects as WASM host imports.

Added

• State<Int>, State<Nat>, State<Bool>, State<Float64> compile to typed host import pairs
• get(()) compiles to call $vera.state_get_T; put(x) compiles to call $vera.state_put_T
• Host runtime maintains mutable state cells per type
• execute() accepts optional initial_state and returns final state
• Mixed effects supported: effects(<State<Int>, IO>)
• 15 new codegen tests (585 total)
• examples/increment.vera now compiles and runs

PR: #38