tidy: fieldtest follow-ups — domain prose for the zero bound, gate-emptied exit semantics documented

The two fieldtest findings, resolved:

- friction: --parallel-instruments 0 now refuses in domain terms ("must be
  a whole number of at least 1 — it bounds how many distinct instruments
  are resident in parallel") via a custom clap value-parser, instead of
  leaking Rust's NonZeroUsize wording; the zero-bound test pins the prose.
- spec_gap: the authoring guide's exit-code contract now states that a
  gate-emptied cell (a std::gate leaving no survivors) is a successful
  cell — exit 0 with an informational aura: note and a truncated
  realization — and that exit 3 is reserved for cells that could not be
  evaluated. Records observed behaviour; no code change.

Gates re-verified: workspace suite green, clippy -D warnings clean.

refs #277

crates/aura-cli/src/research_docs.rs

@@ -60,6 +60,17 @@ impl DocIntrospectCmd {
     }
 }
 
+/// Parse `--parallel-instruments` in domain terms: clap's built-in
+/// `NonZeroUsize` parser would leak the Rust type name ("number would be
+/// zero for non-zero type") at exactly the moment a user needs guidance.
+fn parse_parallel_instruments(s: &str) -> Result<std::num::NonZeroUsize, String> {
+    s.parse::<usize>().ok().and_then(std::num::NonZeroUsize::new).ok_or_else(|| {
+        "must be a whole number of at least 1 — it bounds how many distinct \
+         instruments are resident in parallel"
+            .to_string()
+    })
+}
+
 /// Exactly one introspect mode (the graph-introspect guard idiom: usage
 /// error on stderr, exit 2).
 fn guard_one_mode(cmd: &DocIntrospectCmd, family: &str) {
@@ -320,7 +331,11 @@ pub enum CampaignSub {
         target: String,
         /// Bound on distinct instruments resident in parallel (the RAM
         /// lever; 1 reproduces the sequential loop's footprint).
-        #[arg(long, default_value_t = aura_campaign::DEFAULT_PARALLEL_INSTRUMENTS)]
+        #[arg(
+            long,
+            default_value_t = aura_campaign::DEFAULT_PARALLEL_INSTRUMENTS,
+            value_parser = parse_parallel_instruments,
+        )]
         parallel_instruments: std::num::NonZeroUsize,
     },
     /// List stored campaign realizations, or dump one campaign's records