Minimal XetRuntime for WASM

[seanses]

XetRuntime only needs to be a extremely simple wrapper around tokio_with_wasm for WASM target. This PR

• moves the original runtime.rs to runtime_native.rs and removing all cfg-gated code for WASM target,
• implement a minimal XetRuntime for WASM, and
• add a runtime.rs to expose either the native XetRuntime or the WASM XetRuntime through cfg-gate.

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Note

Medium Risk
Touches core async execution and session bootstrap on both native and WASM; behavior should be equivalent on native after a large file move, but WASM/runtime wiring errors could break browser builds or bridging.

Overview
Splits XetRuntime into target-specific modules: runtime/native.rs holds the full Tokio-backed implementation (moved out of the monolithic runtime.rs), runtime/wasm.rs exposes a minimal stub (no Tokio handle, no-op SIGINT/shutdown, spawn_blocking via tokio_with_wasm), and runtime.rs only defines RuntimeMode and re-exports the right XetRuntime per cfg.

XetContext and XetSessionBuilder::build now use #[cfg] so WASM always constructs the stub via XetContext::with_config and never tries external-handle detection or from_external. Native-only APIs (handle_meets_requirements, from_external) and related session tests are gated off on WASM.

Also ignores .superpowers/ and switches xet_data’s optional pyo3 dep to the workspace definition.

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