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/data.rs

@@ -425,6 +425,24 @@ const LIMIT_CLAUSE_NODES: &[Node] = &[
 ];
 const LIMIT_CLAUSE: Node = Node::Seq(LIMIT_CLAUSE_NODES);
 
+// =================================================================
+// seed — `seed <T> [<count>]` (ADR-0048, SD1)
+// =================================================================
+
+/// Optional positional row count. Reuses `LIMIT_VALIDATOR` (a
+/// non-negative integer). Phase 1 has no `--seed` flag, `set` clause,
+/// or `<table>.<column>` column-fill form yet.
+const SEED_COUNT: Node = Node::NumberLit {
+    validator: Some(LIMIT_VALIDATOR),
+};
+const SEED_NODES: &[Node] = &[
+    // `writes_table` so a future `set <col>=…` clause's column slots
+    // can resolve against this table.
+    TABLE_NAME_WRITES,
+    Node::Optional(&SEED_COUNT),
+];
+const SEED_SHAPE: Node = Node::Seq(SEED_NODES);
+
 const UPDATE_NODES: &[Node] = &[
     TABLE_NAME_WRITES,
     Node::Word(Word::keyword("set")),
@@ -708,6 +726,38 @@ fn build_show_limit(path: &MatchedPath) -> Result<Option<u64>, ValidationError>
         })
 }
 
+/// Build a `seed <T> [<count>]` command (ADR-0048). The only
+/// `NumberLit` in a `seed` path is the optional count.
+fn build_seed(path: &MatchedPath, _source: &str) -> Result<Command, ValidationError> {
+    Ok(Command::Seed {
+        table: require_ident(path, "table_name")?,
+        count: build_seed_count(path)?,
+        // `--seed <n>` is added in a later phase; reproducibility off
+        // for now.
+        rng_seed: None,
+    })
+}
+
+fn build_seed_count(path: &MatchedPath) -> Result<Option<u64>, ValidationError> {
+    let Some(item) = path
+        .items
+        .iter()
+        .find(|i| matches!(i.kind, MatchedKind::NumberLit))
+    else {
+        return Ok(None);
+    };
+    item.text
+        .parse::<u64>()
+        .map(Some)
+        .map_err(|_| ValidationError {
+            message_key: "parse.custom.bind_type_mismatch",
+            args: vec![
+                ("found", item.text.clone()),
+                ("expected", "non-negative integer".to_string()),
+            ],
+        })
+}
+
 fn build_insert(path: &MatchedPath, _source: &str) -> Result<Command, ValidationError> {
     let table = require_ident(path, "table_name")?;
 
@@ -1452,6 +1502,14 @@ pub static SHOW: CommandNode = CommandNode {
         "parse.usage.show_index",
     ],};
 
+pub static SEED: CommandNode = CommandNode {
+    entry: Word::keyword("seed"),
+    shape: SEED_SHAPE,
+    ast_builder: build_seed,
+    help_id: Some("data.seed"),
+    usage_ids: &["parse.usage.seed"],
+};
+
 pub static INSERT: CommandNode = CommandNode {
     entry: Word::keyword("insert"),
     shape: INSERT_SHAPE,