refactor(sdk): add Fault constructors, reject zero interval

[mfw78]

What

Adds inherent constructors on Fault in crates/nexum-sdk/src/host.rs, one per free-text variant: Fault::unsupported, unavailable, denied, invalid_input, internal, each taking impl Into<String>. RateLimited and Timeout are unit variants and get none. The three module-local lifters are deleted and their call sites moved onto the constructors: balance-tracker's invalid_input, http-probe's invalid_input and internal, and price-alert's invalid. http-probe's inline Fault::InvalidInput(detail) in the FetchError arm goes through the constructor too, for consistency; the arm itself is unchanged.

price-alert no longer clamps every_n_blocks = 0 up to 1. It now refuses with every_n_blocks must be >= 1, matching http-probe, and a new test pins the refusal.

Why

Closes #140. Every example module reaches the SDK config conversion through config::get_required(...)?, so the only thing the local helpers still did was lift a free-text detail, and three copies had drifted into two names and two signatures. Per the issue's settled decisions the lifter becomes inherent constructors on Fault, where the type lives, discoverable by autocomplete and idiomatic; all five free-text variants get one so the rule is uniform rather than arbitrary.

A zero interval is rejected in both throttling modules rather than clamped. It fails loudly at init while the operator is watching, instead of producing a module that looks configured and quietly does the maximum work. Removing the clamp in favour of nothing was never on the table: is_multiple_of(0) is true only for block 0, so an unclamped zero would silently mean "never fire". Blast radius is bounded to the two example component.toml files, both of which set 1.

Testing

• cargo fmt --all, cargo clippy --workspace --all-targets --all-features --locked -- -D warnings, clean
• just build, then cargo nextest run --workspace --all-features --locked, 858 passed, 0 skipped
• ./scripts/workspace-deps-lint.sh and cargo machete, both clean
• New: parse_config_rejects_zero_every_n_blocks in price-alert. http-probe's parse_config_rejects_missing_urls_and_zero_throttle is untouched.

AI Assistance

Claude Opus 5 used for the constructor rollout, the call site migration and the price-alert refusal test.