feat(host): switch the generic interfaces to fault end to end

[mfw78]

Summary

Closes #208 (part of the typed-fault train stacked on the epic-80 stack, epic #206; merges in stack order after #213, feat/fault-vocabulary).

Moves the four generic host interfaces (local-store, remote-store, identity, messaging) from the unified host-error envelope to the typed fault vocabulary end to end: WIT signatures, host implementations, the SDK host trait and macro conversion ladder, mocks, and the one raw wit-bindgen module fixture that needs its own converter.

Changes

Each of wit/nexum-host/{local-store,remote-store,identity,messaging}.wit moves every function from result<_, host-error> to result<_, fault> and pulls in use types.{fault} (messaging keeps message alongside it).
identity.wit also updates its doc comment: rejection is now described as fault.denied.
Chain, the event-module and query-module exports, and logging are untouched, since they are out of scope for this sub-issue.

crates/nexum-runtime/src/host/error.rs retargets From<StorageError> from HostError to Fault (Fault::Internal carrying err.to_string()), with the module doc reworded to describe both shapes side by side.
HostError constructors stay live, since chain and cow-api impls still return them.

The four host impls under crates/nexum-runtime/src/host/impls/ return the Fault associated error type.
Local-store maps StorageError through the retargeted From; the three deferred backends (remote-store, identity, messaging) return Fault::Unsupported carrying the same detail string the prior unimplemented() used.
accounts/query continue to return an empty Ok.

crates/nexum-sdk/src/host.rs changes LocalStoreHost to Result<_, Fault> and adds impl From<Fault> for HostError, which is the bridge that lets module strategies still returning a unified HostError call the now-fault-typed store methods with a mechanical ?.
The Host doctest is updated to match.

crates/nexum-sdk/src/wit_bindgen_macro.rs adds convert_fault, mapping the wit-bindgen-generated Fault to the SDK Fault (including the RateLimit field), and routes the LocalStoreHost impl through it.
Macro-level docs list the new symbol.

crates/nexum-sdk/src/chain/chainlink.rs updates the test StubHost store methods to return Fault.

crates/nexum-sdk-test/src/lib.rs and crates/shepherd-sdk-test/src/lib.rs move MockLocalStore/MockHost store methods, fail_on, check_injected_error, error_patterns, and the max-entries error path onto Fault, with the two crate-internal unit tests updated to match.

modules/fixtures/flaky-bomb/src/lib.rs is a raw wit-bindgen module without the SDK macro, so it gets its own small store_fault helper that folds the local-store fault back into the export host-error so the counter reads and writes still compose with ?.

Test plan

• cargo fmt --all --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo doc --workspace --no-deps with RUSTDOCFLAGS=-D warnings
• All 11 module wasms rebuilt for wasm32-wasip2 --release, in-tree, so supervisor e2e/boot tests resolve fresh artefacts
• cargo test --workspace --all-features
• cargo test -p nexum-runtime --all-features (196 passed, 0 failed, 0 skipped), including supervisor::tests::restart_flaky_module_recovers_after_backoff, which drives the flaky-bomb guest's local_store::get/set across the wasm boundary against the new result<_, fault> wire type

Notes for reviewer

From<Fault> for HostError in nexum-sdk is new and is the one deliberate aggregation seam in this change.
Module strategies (twap-monitor, ethflow-watcher, balance-tracker, stop-loss) each return a single Result<_, HostError> while mixing a now-fault-typed store call with still-host-error chain and cow-api calls, so the bridge keeps ? mechanical across roughly thirty call sites without touching their signatures.
It disappears once chain, cow-api, and the export boundary also migrate to fault.

Fault deliberately omits domain/code.
Every prior detail string is preserved verbatim in the fault payload (deferred-backend messages, StorageError display output, mock injected-error messages); the dropped fields were always static per-interface constants that are redundant once the interface itself is the WASI domain tag.

flaky-bomb hand-rolls its converter because it uses raw wit-bindgen rather than the SDK macro, and keeps domain: "local-store" since it constructs the export HostError directly.

Left for a follow-up: prose in docs/00-overview.md, docs/07-rpc-namespace-design.md, docs/08-platform-generalisation.md, docs/adr/0009, and docs/migration/0.1-to-0.2.md still describes a single unified host-error model. A partial edit here would leave those pages internally inconsistent mid-migration, so a coherent doc pass is deferred until chain, cow-api, and the exports also move.

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