fix(observability): name the module on the chain-request lines

[mfw78]

What

Four tracing sites in the chain host call now name the module: the response-cap refusal, the denied-method refusal, and the two chain::request lines below the default level. check_response_cap takes the module rather than deriving it, since it is a free function.

The runbook query in docs/production.md is fixed. It filtered on .fields.module, but crates/nexum-launch/src/lib.rs sets flatten_event(true), so event fields sit at the top level and there is no .fields object. The published command matched nothing on every line.

Why

Two of the four print at the default log_level = "info" and are the only lines that answer "which module". nexum_runtime_chain_request_total carries chain_id, method and outcome but no module label, and docs/production.md sells method="<denied>" as the signal that a module is reaching outside the read surface. Neither the counter nor the log line named which one, so that alert was undiagnosable from telemetry alone. Same hole on nexum_runtime_chain_response_capped_total.

The runbook fix is unrelated and pre-existing. It is here because it is one line and was found in the same review.

Why not a span

#373 proposed a dispatch span carrying the module, and is closed unmerged. Four lenses agreed it did not earn the change: every tracing site in supervisor/dispatch.rs already carries module, as do the guest-mirror sites, the http gate and the store error path, so the span would have duplicated a field already on the line. It carried no span id, so it did not distinguish two concurrent dispatches of one module either. These four sites were its entire verified benefit.

Passing the field also covers a case the span could not: instantiate_module runs guest init inside no span, and a host call made from init reaches these same seams.

Testing

cargo nextest run -p nexum-runtime-wasm --all-features --locked: 28 passed. Clippy over the crate with -D warnings clean, cargo fmt --all --check clean, content-lint.sh ok, just build green.

AI Assistance

Review by four claude-opus-5 lenses; fix and PR description by claude-opus-5.