feat(seed): command plumbing + walking skeleton (ADR-0048 P1.2)

End-to-end `seed <table> [count]` path, both modes:
- Command::Seed AST + grammar node (show-data table slot + optional
  positional count) + REGISTRY registration + build_seed.
- Runtime dispatch -> Database::seed -> Request::Seed worker arm ->
  do_seed.
- do_seed (Phase-1 skeleton): generates whole rows for non-FK,
  non-autogen columns via the seed library and inserts them one at a
  time through do_insert (reusing validation / autogen autofill /
  FK-error / persistence). One undo step (snapshot_then wraps it) and
  one history.log line (only the first row carries the source);
  default count 20.
- help (`help seed`) + parse-usage catalog entries.
- Reuses CommandOutcome::Insert for the auto-show; a dedicated
  SeedResult (capped preview + advisory) replaces it in P1.3.

5 Tier-3 integration tests (parse, populate+persist, default-20,
reproducible --seed, one history line). 2327 pass / 0 fail / 0 skip,
clippy all-targets clean.

Deferred to P1.3: FK sampling, identifier/constraint uniqueness, CHECK
derivation, block guard, capped preview, advisory, multi-row path.
Deferred to P1.4: completion/highlight/hint/validity wiring + --seed flag.

src/runtime.rs

@@ -2911,6 +2911,17 @@ async fn execute_command_typed(
             .insert(table, columns, values, src)
             .await
             .map(CommandOutcome::Insert),
+        // ADR-0048 (SD1). Phase 1 reuses the insert outcome for the
+        // auto-show; a dedicated `SeedResult` (capped preview +
+        // enum/CHECK advisory) replaces this in a later phase.
+        Command::Seed {
+            table,
+            count,
+            rng_seed,
+        } => database
+            .seed(table, count, rng_seed, src)
+            .await
+            .map(CommandOutcome::Insert),
         Command::Update {
             table,
             assignments,