feat(seed): --seed flag, ambient wiring, and /runda hardening (ADR-0048 P1.4 + DA)

P1.4 — user-visible surface:
- Grammar: `seed <table> [count] [--seed <n>]` (the first DSL flag with a
  value); build_seed disambiguates the seed value from the positional count.
- Verified the auto-wired surface: table-name completion, --seed offered as
  a candidate, validity consistent with `show data`, an ADR-0042 near-miss
  row for bare `seed`, and render tests for the seed outcome.

/runda hardening — eight DA findings, all resolved:
- FK sampling now uses ORDER BY so --seed reproducibility no longer relies
  on SQLite's unspecified DISTINCT order (D4).
- shortid columns now generate from seed's seeded RNG (new
  shortid::generate_with_rng) — D4 now holds with no exceptions.
- Added the missing coverage the DA flagged: undo-one-step (D15), replay
  re-runs a seed line (D16), advanced-mode (D5), atomic rollback on a
  constraint failure, seed 0 no-op, complex-CHECK advisory (D17), and
  FK + shortid reproducibility.

2358 pass / 0 fail / 0 skip, clippy all-targets clean.

tests/it/parse_error_pedagogy.rs

@@ -109,6 +109,7 @@ fn near_miss_matrix_simple_mode() {
         ("delete", &["after `delete`, expected `from`", "delete from <Table>"]),
         ("delete from", &["after `delete from`, expected table name", "delete from <Table>"]),
         ("delete from T", &["expected `where` or `--all-rows`", "delete from <Table>"]),
+        ("seed", &["after `seed`, expected table name", "seed <Table> [count]"]),
         ("replay", &["after `replay`, expected string literal or path", "replay <path>"]),
         ("explain", &["after `explain`, expected `show`, `update`, or `delete`", "explain show data"]),
         // advanced-only entry word typed in simple mode → "this is SQL" rail