feat(wit-abi): add the fault vocabulary and SDK HostFault surface

[mfw78]

Summary

Closes #207, part of the typed-fault train stacked on the epic-80 stack, merges in stack order.

Adds the shared fault vocabulary that later host interfaces will embed as a case, plus the SDK-side mirror that module authors program against.

Changes

• wit/nexum-host/types.wit: added variant fault with the seven specified cases (unsupported, unavailable, denied, rate-limited, timeout, invalid-input, internal) and record rate-limit { retry-after-ms: option<u64> }, both alongside the existing untouched host-error record.
• crates/nexum-sdk/src/host.rs: mirrored Fault (thiserror #[error] per case, IntoStaticStr with snake_case labels, #[non_exhaustive]), the RateLimit struct, and the HostFault trait (fault(&self) -> Option<&Fault>, label(&self) -> &'static str) with a reflexive impl on Fault itself.
• Rebuilt the module wasms in-tree (example, twap-monitor, ethflow-watcher, price-alert, balance-tracker, stop-loss, http-probe, flaky-bomb, fuel-bomb, memory-bomb, panic-bomb) so the supervisor e2e tests resolve them, since the wit path was touched.

No host interface or consumer moves onto Fault yet and host-error is not removed. That migration is deliberately out of scope here and lands in the next PRs in the stack.

Test plan

• cargo fmt --all --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-features --no-fail-fast (new tests host::tests::fault_labels_are_stable_snake_case and host::tests::host_fault_is_object_safe both pass)
• wasm-tools component wit wit/nexum-host round-trips the package with the new variant and record present

Notes for reviewer

RateLimit derives Copy, Default, and Hash since it is a single Option<u64>; Fault itself stays Clone only because it carries String payloads.

label() delegates to IntoStaticStr via self.into() so the snake_case set has one source of truth, and the test pins all seven labels plus the rate_limited underscore form.

Object safety is proven rather than asserted: host_fault_is_object_safe boxes a dyn HostFault so both trait methods are checked as dispatchable.

Follow-ups intentionally not implemented here: migrating host interfaces onto typed per-interface errors that embed Fault, retiring host-error once every consumer has moved, and adding From/TryFrom between the wit-bindgen-generated Fault and the SDK Fault once the first consumer interface lands.

There is one pre-existing duplicated_attributes clippy lint in nexum-sdk's proptests.rs/lib.rs that only surfaces under the host's rustc 1.92 rather than the project-pinned 1.94; it is untouched by this diff and present on the base commit.

AI Assistance: Claude Code (Opus 4.8 implementation and adversarial review, Sonnet 5 PR authoring) used for the full change.