Add tracing spans and structured fields to Ninja subprocess execution

[coderabbitai]

Background

The NETSUKE_NINJA feature (PR #323) improved observability around Ninja subprocess execution. The following have already been implemented in PR #323:

• command_span creates a "ninja_subprocess" tracing span with stable low-cardinality fields: operation, ninja_program, redacted_arg_count, suppress_stderr, failure_category = field::Empty
• Spawn and exit failures record failure_category = "spawn" and failure_category = "exit_status" as structured span fields
• log_command_execution, log_command_spawn_failure, log_command_exit_failure emit structured tracing events with operation and ninja_program fields

Remaining required work

Two acceptance criteria from the original description remain unmet:

1. Log NETSUKE_NINJA resolution at the env-var boundary

resolve_ninja_program_utf8_with in src/runner/process/mod.rs silently picks the env-var value or falls back to NINJA_PROGRAM with no trace event. Add a debug! event at that boundary, e.g.:

fn resolve_ninja_program_utf8_with<F>(mut read_env: F) -> Utf8PathBuf
where
    F: FnMut(&str) -> Option<OsString>,
{
    match read_env(NINJA_ENV).and_then(|value| {
        Utf8PathBuf::from_path_buf(PathBuf::from(value)).ok()
    }) {
        Some(path) => {
            tracing::debug!(
                ninja_program = %path,
                source = "env",
                "Resolved Ninja program from {NINJA_ENV}"
            );
            path
        }
        None => {
            tracing::debug!(
                ninja_program = NINJA_PROGRAM,
                source = "default",
                "NETSUKE_NINJA unset or non-UTF-8; using default Ninja program"
            );
            Utf8PathBuf::from(NINJA_PROGRAM)
        }
    }
}

2. Demote high-cardinality args/paths from the log event message

log_command_execution currently emits the full redacted_command string (which includes the build file path and target list) in the message body. Move high-cardinality data to a debug! companion event rather than including it in the info! span event message. The info! event should carry only stable low-cardinality fields; the redacted_command string should be emitted separately at debug! level.

Acceptance criteria

• resolve_ninja_program_utf8_with emits a debug! event at the env-var resolution boundary with ninja_program and source fields
• log_command_execution no longer includes redacted_command in its info! message body; that string is emitted at debug! level instead
• Existing tests continue to pass under make check-fmt && make lint && make test
• No broad lint suppressions beyond narrowly scoped #[expect(..., reason = "...")]

Backlinks

• PR: Document Ninja executable override (#83) #323
• Issue: Support overriding Ninja executable via NETSUKE_NINJA #83
• Raised by: @leynos

[coderabbitai]

🔗 Related PRs

#302 - Overhaul Netsuke CLI documentation contract [merged]
#305 - Add Kani tooling and local smoke targets (4.1.1) [merged]
#308 - Add Kani smoke CI job (4.1.2) [merged]
#309 - Expand top-level action flow control (3.14.2) [merged]
leynos/mdtablefix#303 - Add observability for inline wrapping token classification (#296) [merged]

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