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/command.rs

@@ -402,6 +402,16 @@ pub enum Command {
         filter: Option<Expr>,
         limit: Option<u64>,
     },
+    /// Populate a table with generated fake data (ADR-0048, SD1).
+    /// `count` defaults to 20 when omitted; `rng_seed` (from a future
+    /// `--seed <n>` flag) makes generation reproducible. Phase 1 is
+    /// whole-row generation; the `set` override clause and the
+    /// `<table>.<column>` column-fill form arrive in later phases.
+    Seed {
+        table: String,
+        count: Option<u64>,
+        rng_seed: Option<u64>,
+    },
     /// Replay a sequence of DSL commands from a file. Each line
     /// is parsed and dispatched through the same pipeline as
     /// interactive input. Blank lines and lines whose first
@@ -949,6 +959,7 @@ impl Command {
             } => "show index",
             Self::ShowList { kind, .. } => kind.command_name(),
             Self::Insert { .. } => "insert into",
+            Self::Seed { .. } => "seed",
             Self::Update { .. } => "update",
             Self::Delete { .. } => "delete from",
             Self::ShowData { .. } => "show data",
@@ -997,6 +1008,7 @@ impl Command {
             | Self::AddConstraint { table, .. }
             | Self::DropConstraint { table, .. }
             | Self::Insert { table, .. }
+            | Self::Seed { table, .. }
             | Self::Update { table, .. }
             | Self::Delete { table, .. } => table,
             // For relationships we focus on the parent (1-side):