feat(seed): SeedResult outcome, capped preview, advisory, count cap (ADR-0048 P1.3c)

A dedicated SeedResult replaces the borrowed insert outcome (X5):
- CommandOutcome::Seed + DslSeedSucceeded event + handle_dsl_seed_success
  render: the echo, "N row(s) seeded into T", a capped preview table
  (D18, first 20 rows; full count always reported), and a Hint-styled
  advisory naming enum-ish / un-derivable-CHECK columns filled with
  generic text (D12/D13, Phase-1 wording).
- SeedResult carries requested vs produced, so a junction cap is now
  reported to the user, not only logged.
- Count cap (D6): a seed over 10000 rows is refused with a friendly error.
- Catalog keys ok.rows_seeded / seed.capped / seed.advisory_generic.

4 new tests (advisory flag, IN-check not flagged, preview cap, excess
count). 2346 pass / 0 fail / 0 skip, clippy clean.

src/event.rs

@@ -87,6 +87,10 @@ pub enum AppEvent {
         command: Command,
         result: InsertResult,
     },
+    DslSeedSucceeded {
+        command: Command,
+        result: crate::db::SeedResult,
+    },
     DslUpdateSucceeded {
         command: Command,
         result: UpdateResult,