feat(research,campaign,cli): std::grid — the enumerate-only leading stage

closes #256

Fork B (owner decision 2026-07-14): the dissolved walkforward/mc
translations' leading sweep executed the full grid over the whole campaign
window and persisted a Sweep family, yet only the enumerated parameter
points ever crossed the stage seam (the wf stage re-sweeps them per IS
window itself). The leading stage is now the fieldless vocabulary block
std::grid: it enumerates, executes nothing, persists nothing.

- aura-research: StageBlock::Grid ({"block":"std::grid"}), schema-strict
  parse arm (empty slot list: every key but "block" is refused by the
  generic unknown-slot check), PROCESS_BLOCKS entry, intrinsic-tier no-op
  arm; the vocabulary test's non-empty-slots guard carries a pinned
  std::grid-only exception (a nominal slot would misdescribe the
  vocabulary to describe_block consumers).
- aura-campaign: the inter-stage seam is a typed two-armed StageFlow
  (points-only vs executed members); gate / mc-per-survivor fence the
  points-only arm with defensive PipelineShape faults; preflight admits
  std::grid only as the first stage and only immediately before
  std::walk_forward (every other neighbor consumes executed reports).
- aura-cli: translate_walkforward / translate_mc lead with
  StageBlock::Grid; the two family-shape E2E pins flip to zero Sweep
  families; translate_generalize keeps its executed sweep(argmax) — its
  generalize stage consumes the argmax winner report as the cell nominee.
- docs: dated #256 amendment in the ledger's verb-dissolution narrative;
  the authoring-guide vocabulary transcript gains the std::grid line.

Behaviour preservation: the exact-grade real-data pins
(walkforward_real_e2e_pins_the_exact_current_grade,
mc_r_bootstrap_real_e2e_pins_the_exact_current_grade) pass unmodified —
survivor points reach the wf stage in the same odometer order as before.

Measured (the #256 acceptance measurement; debug build, real GER40 2025,
2x2 grid, `aura walkforward --real`, 3 runs each):
  before (a55e4cf): 6.27 / 6.18 / 6.18 s
  after:            4.52 / 4.49 / 4.45 s   (~ -27% wall clock)

Suite: cargo test --workspace green (0 failed); clippy clean.
Decision log: #256 comments (fork rationale incl. the StageFlow seam and
the slot-guard exception).

crates/aura-cli/tests/cli_run.rs

@@ -3293,8 +3293,8 @@ fn walkforward_dissolved_refuses_name_and_trace_together() {
 /// generated campaign document (carrying the `--name` handle and the stop as a
 /// non-empty single risk regime), and one campaign-run record — mirroring the
 /// generalize dissolution's audit trail. The persisted family set is exactly one
-/// `Sweep` family (the pipeline's leading `std::sweep(argmax)` stage) plus one
-/// `WalkForward` family (the per-window OOS reports) — the observable proof that
+/// `WalkForward` family (the per-window OOS reports) and NO `Sweep` family —
+/// the leading `std::grid` stage only enumerates (#256) — the observable proof that
 /// `dispatch_walkforward`'s real-data path runs through the one campaign
 /// executor rather than the deleted inline roller. Gated on the shared GER40
 /// archive; skips cleanly on a data refusal.
@@ -3363,8 +3363,8 @@ fn walkforward_dissolves_through_the_campaign_path() {
     let fams_out = String::from_utf8_lossy(&fams.stdout).into_owned();
     assert_eq!(
         fams_out.lines().filter(|l| l.contains("\"kind\":\"Sweep\"")).count(),
-        1,
-        "one Sweep family from the pipeline's leading argmax stage: {fams_out}"
+        0,
+        "the enumerate-only std::grid leading stage persists no Sweep family: {fams_out}"
     );
     assert_eq!(
         fams_out.lines().filter(|l| l.contains("\"kind\":\"WalkForward\"")).count(),
@@ -5312,9 +5312,10 @@ fn mc_dissolves_a_non_r_sma_blueprint() {
 /// path — a successful run durably auto-registers exactly one generated process
 /// document, one generated campaign document (carrying the constant "mc" name and the
 /// stop as a non-empty single risk regime), and one campaign-run record, and emits the
-/// single `mc_r_bootstrap` grade line. The pipeline's leading argmax sweep and its
-/// walk_forward stage each persist one family; the terminal monte_carlo stage is an
-/// annotator and adds NO family. This is the observable proof the inline path is gone
+/// single `mc_r_bootstrap` grade line. The pipeline's walk_forward stage persists
+/// the one family; the leading `std::grid` stage only enumerates (#256) and the
+/// terminal monte_carlo stage is an annotator — neither adds a family. This is
+/// the observable proof the inline path is gone
 /// and the dissolution runs through the executor. Gated on the GER40 2025 archive;
 /// skips cleanly on a data refusal.
 #[test]
@@ -5391,8 +5392,8 @@ fn mc_dissolves_through_the_campaign_path() {
     let fams_out = String::from_utf8_lossy(&fams.stdout).into_owned();
     assert_eq!(
         fams_out.lines().filter(|l| l.contains("\"kind\":\"Sweep\"")).count(),
-        1,
-        "one Sweep family from the pipeline's leading argmax stage: {fams_out}"
+        0,
+        "the enumerate-only std::grid leading stage persists no Sweep family: {fams_out}"
     );
     assert_eq!(
         fams_out.lines().filter(|l| l.contains("\"kind\":\"WalkForward\"")).count(),
@@ -5416,9 +5417,9 @@ fn mc_dissolves_through_the_campaign_path() {
 /// data-carrying window far shorter than the roller", which a fixed recent window
 /// satisfies without the full-archive probe sweep a live-span derivation costs —
 /// the archive only grows forward, so the window never falls off its end. The
-/// leading sweep stage runs first over the short window; the refusal today fires at
-/// stage 1 (the walk_forward roller), so this bites only when the window genuinely has
-/// data. Gated on the GER40 archive; skips cleanly on a data-less host.
+/// leading std::grid stage only enumerates (nothing runs over the short window); the
+/// refusal fires at stage 1 (the walk_forward roller), so this bites only when the
+/// window genuinely has data. Gated on the GER40 archive; skips cleanly on a data-less host.
 #[test]
 fn mc_real_fits_the_injected_roller_to_a_short_window() {
     if !local_data_present() {