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/theme.rs

@@ -163,6 +163,7 @@ impl Theme {
             HighlightClass::String => self.tok_string,
             HighlightClass::Punct => self.tok_punct,
             HighlightClass::Flag => self.tok_flag,
+            HighlightClass::Function => self.tok_function,
             HighlightClass::Error => self.tok_error,
         }
     }
@@ -228,6 +229,7 @@ mod tests {
         assert_eq!(t.highlight_class_color(HighlightClass::String), t.tok_string);
         assert_eq!(t.highlight_class_color(HighlightClass::Punct), t.tok_punct);
         assert_eq!(t.highlight_class_color(HighlightClass::Flag), t.tok_flag);
+        assert_eq!(t.highlight_class_color(HighlightClass::Function), t.tok_function);
         assert_eq!(t.highlight_class_color(HighlightClass::Error), t.tok_error);
     }