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

docs/authoring-guide.md

@@ -609,4 +609,9 @@ Exit codes: a clean run exits 0; a usage error exits 2; a refusal before any
 cell runs (invalid document, missing project, unresolvable strategy) exits 1;
 a run that **completes with one or more failed cells** exits 3 — the run record
 and every healthy cell persist, and the failed cells are named on stderr
-(#272).
+(#272). A **gate-emptied cell** — a `std::gate` stage that filters out every
+member — is a *successful* cell, not a failed one: the gate legitimately
+answered "no survivors", so the run still exits 0, the cell's realization is
+recorded as truncated at that stage, and an informational `aura:`-prefixed
+note names the cell on stderr. Exit 3 is reserved for cells that could not be
+evaluated (faults), never for an empty-but-valid result.