feat(cli): stderr class markers — diag module, campaign/verb-sugar retags

Iteration stderr-markers-1, tasks 1-2 of the stderr-class-markers plan
(spec signed via grounding-check PASS, decisions logged on #278).

The two stderr diagnostic classes become machine-separable (refs #278):
a new crate-internal diag module owns the grammar as note!/warning!
macros (`aura: note: <text>` benign continuing-run diagnostic,
`aura: warning: <text>` recorded fault the run survives); the four
undifferentiated sites are retagged — gate-emptied cell -> note,
failed cell / walkforward cell / mc cell -> warning — and the four
scaffold literals migrate onto the shared macro (output bytes
unchanged). The always-on record summary line gains the bare `aura: `
namespace it was missing; bare-prefix stays the vocabulary for error
lines accompanying a non-zero exit (C14 partition) and plain info
lines. Exit codes unchanged throughout.

Verification: RED->GREEN warning pin on the per-cell-fault e2e; new
gate-emptied e2e (impossible n_trades threshold) pins the note marker,
exit 0, and the summary-line prefix; scaffold e2es and the full
workspace suite green; clippy -D warnings clean on aura-cli. The C28
shell-module roster test gained the diag.rs entry.

Known residue for the next slices: aura-runner literals (stale-dylib
warning, three tap/no-nominee notes) and the #313 zero-trade notice
follow in tasks 3-5. The verb_sugar warning call sites remain e2e-
uncovered (a hostless walkforward fault fixture is structurally
unreachable — single-instrument validation refuses before the cell
loop); their grammar is pinned via the shared macro and the campaign
warning e2e.

refs #278

crates/aura-vocabulary/tests/c28_layering.rs

@@ -179,6 +179,7 @@ fn the_shell_defines_no_domain_modules_and_no_lib_target() {
         modules,
         [
             "campaign_run.rs",
+            "diag.rs",
             "graph_construct.rs",
             "main.rs",
             "render.rs",