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.

src/seed/generators.rs

@@ -100,7 +100,7 @@ fn generic_for_type(ty: Type, rng: &mut SeedRng) -> Value {
             let words: Vec<String> = lorem::Words(2..4).fake_with_rng(rng);
             Value::Text(words.join(" "))
         }
-        Type::ShortId => Value::Text(crate::dsl::shortid::generate()),
+        Type::ShortId => Value::Text(crate::dsl::shortid::generate_with_rng(rng)),
         Type::Int => Value::Number(rng.random_range(1..=10_000).to_string()),
         Type::Serial => Value::Number(rng.random_range(1..=10_000).to_string()),
         Type::Real => {