sdk: From/TryFrom between the wit-bindgen Fault and the SDK Fault

[mfw78]

Problem. A ChainError -> Fault conversion exists (host.rs), but there is no From / TryFrom between the wit-bindgen-generated Fault and the SDK Fault; call sites convert by hand.

Change. Add From / TryFrom between the bindgen Fault and the SDK Fault.

Relates. nullislabs/shepherd#206. Surfaced by the nullislabs/shepherd#213 review notes.

[lgahdl]

Resolved as part of nullislabs/shepherd#358 (fixing nullislabs/shepherd#264): bind_host_via_wit_bindgen! now emits impl From<nexum::host::types::Fault> for $crate::host::Fault and the reverse, replacing the old convert_fault/sdk_fault_into_wit named-function shims — exactly the From this issue asked for. Will auto-close on nullislabs/shepherd#358's merge.

[mfw78]

The keep-list in #50 is explicit that per-cdylib bindgen types forbid a shared From: each cdylib owns its own generated Fault, and the lowering between the wit-bindgen Fault and the SDK Fault is the ratified ABI-boundary pattern, so the shared conversion this issue requests cannot exist as written.

The remaining options are to rescope this to a macro-emitted per-cdylib conversion inside wit_bindgen_macro, or to close it as superseded by #50's keep-list. That decision is deferred to #50's implementation in M2, and the two must not run independently.