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/dsl/grammar/mod.rs

@@ -57,6 +57,12 @@ pub enum HighlightClass {
     String,
     Punct,
     Flag,
+    /// A curated function-vocabulary name — the `seed … set <col> as
+    /// <generator>` generator names (ADR-0048 D2/§Grammar). Rendered in
+    /// the existing `tok_function` colour (ADR-0022 Amд6 blue — no new
+    /// theme colour), assigned via a generator slot's
+    /// `highlight_override`, not by byte shape.
+    Function,
     Error,
 }
 
@@ -86,6 +92,14 @@ pub enum IdentSource {
     /// content validator on column-type slots; not user-listable
     /// from the schema.
     Types,
+    /// Closed, curated set of fake-data generator names (ADR-0048
+    /// D9) — the `seed … set <col> as <generator>` slot. Like
+    /// `Types`, not user-listable from the schema; the vocabulary
+    /// lives in `src/seed` and the completion engine offers it. The
+    /// grammar slot is purely structural (matches any identifier);
+    /// an unknown name is flagged live (validity) and rejected at
+    /// execution.
+    Generators,
     /// Any identifier shape; used by synthetic catch-all branches
     /// (e.g., the unknown-value branch of `mode <value>`).
     Free,
@@ -117,6 +131,7 @@ impl IdentSource {
             Self::Relationships => "relationship name",
             Self::Indexes => "index name",
             Self::Types => "type",
+            Self::Generators => "generator name",
         }
     }
 
@@ -134,6 +149,7 @@ impl IdentSource {
             "relationship name" => Some(Self::Relationships),
             "index name" => Some(Self::Indexes),
             "type" => Some(Self::Types),
+            "generator name" => Some(Self::Generators),
             _ => None,
         }
     }