feat(seed): set override clause + column-fill (ADR-0048 Phase 2)

Build the two SD2 surfaces Phase 1 deferred:

- `set` override clause (D2): comma-separated per-column pins —
  `= 'v'` (fixed), `in ('a','b')` (pick-list), `as <generator>`
  (named), `between x and y` (range; numeric and quoted dates).
  Type-aware via the typed `current_column_value` slot; an override
  drops its column from the generic-fill advisory (D13). Folded from
  the flat matched path (build_seed_overrides) and applied to the
  per-column plan (apply_seed_overrides).
- `<table>.<column>` column-fill (D1 form 2): an UPDATE over existing
  rows. Refuses PK/autogen targets, empty-table no-op, FK-samples the
  parent, collision-free for UNIQUE/identifier targets, one undo step;
  `set` may only adjust the filled column.

Supporting work: KNOWN_GENERATORS vocabulary + generator_for_name
(src/seed/vocabulary.rs, D9); a range Generator + range_bounds_reason;
IdentSource::Generators and HighlightClass::Function; completion of the
generator vocabulary after `as` and the set/.col column slots; the
typing-time validity indicator for an unknown generator; help,
parse-error pedagogy rows, and the D13 advisory's Phase-2/3 wording.

A bounded override (fixed value / too-short pick-list) on a
single-column-UNIQUE target is a friendly error rather than a silent
uniqueness cap (post-implementation /runda finding, user-chosen).

Dates in the range form are quoted (no date-literal token exists);
ADR-0048 D2 amended accordingly. Both modes (D5); reproducible (D4).

src/runtime.rs

@@ -2916,13 +2916,15 @@ async fn execute_command_typed(
             .insert(table, columns, values, src)
             .await
             .map(CommandOutcome::Insert),
-        // ADR-0048 (SD1).
+        // ADR-0048 (SD1/SD2 Phase 2).
         Command::Seed {
             table,
+            target_column,
             count,
+            overrides,
             rng_seed,
         } => database
-            .seed(table, count, rng_seed, src)
+            .seed(table, target_column, count, overrides, rng_seed, src)
             .await
             .map(CommandOutcome::Seed),
         Command::Update {