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).

docs/authoring-guide.md

@@ -350,6 +350,7 @@ std::gate          filter survivors by a conjunction of typed metric predicates
 std::walk_forward  rolling in-sample optimize + out-of-sample test
 std::monte_carlo   R-bootstrap over realised R (terminal annotator): ...
 std::generalize    cross-instrument worst-case floor (terminal annotator): ...
+std::grid          enumerate the axis grid as parameter points for the next stage (enumerate-only leading stage): ...
 $ aura process introspect --block std::sweep
 std::sweep — evaluate the campaign's axes-space; reduce members to R metrics; optionally select a winner (the selection group metric+select is all-or-nothing; omitted = selection-free, terminal-stage-only)
   metric               optional, metric name (see metric_vocabulary)