feat(seed): uniqueness, junction distinct-combos, IN-CHECK (ADR-0048 P1.3b)

do_seed now enforces value uniqueness and derives enum values:
- Uniqueness groups (D10): the user-fillable PK, compound UNIQUE
  constraints, and single-column UNIQUE / identifier columns stay
  distinct across the batch and against existing rows (retry per row).
  Junction distinct-combos fall out of PK-tuple uniqueness and cap at
  the available parent combinations (logged when capped; the
  user-facing note arrives with the advisory in P1.3c).
- Identifier-int columns get a monotonic sequence past MAX(col) (D10),
  so they never collide.
- IN-CHECK derivation (D17): a simple `col IN ('a','b')` CHECK becomes
  the value source via the new, unit-tested seed::parse_in_check_values,
  so the enum-as-CHECK pattern just works.

8 parser unit tests + 4 integration tests (unique column, identifier
sequencing, junction cap, IN-check enum). 2343 pass / 0 fail / 0 skip,
clippy all-targets clean.

Deferred to P1.3c: dedicated SeedResult + capped preview (D18) + the
enum/CHECK advisory incl. the cap note (D12/D13); P1.3d: multi-row path.

src/seed/mod.rs

@@ -24,9 +24,11 @@
 //!   production, the hand-rolled `product` generator (D9) and the
 //!   bounded date windows (D8).
 
+mod check;
 mod generators;
 mod heuristics;
 
+pub use check::parse_in_check_values;
 pub use generators::generate_value;
 pub use heuristics::{choose_generator, is_enum_ish};