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/dsl/grammar/mod.rs

@@ -714,6 +714,7 @@ pub static REGISTRY: &[(&CommandNode, CommandCategory)] = &[
     (&ddl::CREATE, CommandCategory::Simple),
     (&ddl::CREATE_M2N, CommandCategory::Simple),
     (&data::SHOW, CommandCategory::Simple),
+    (&data::SEED, CommandCategory::Simple),
     (&data::INSERT, CommandCategory::Simple),
     (&data::UPDATE, CommandCategory::Simple),
     (&data::DELETE, CommandCategory::Simple),