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/command.rs

@@ -402,14 +402,23 @@ pub enum Command {
         filter: Option<Expr>,
         limit: Option<u64>,
     },
-    /// Populate a table with generated fake data (ADR-0048, SD1).
-    /// `count` defaults to 20 when omitted; `rng_seed` (from a future
-    /// `--seed <n>` flag) makes generation reproducible. Phase 1 is
-    /// whole-row generation; the `set` override clause and the
-    /// `<table>.<column>` column-fill form arrive in later phases.
+    /// Populate a table with generated fake data (ADR-0048, SD1/SD2).
+    /// `count` defaults to 20 when omitted; `rng_seed` (from the
+    /// `--seed <n>` flag) makes generation reproducible.
+    ///
+    /// Phase 2 surfaces (ADR-0048 D1/D2):
+    /// - `target_column` is `Some` for the **column-fill** form
+    ///   `seed <table>.<column>` — fill one column across the table's
+    ///   *existing* rows (an UPDATE), rather than generating new rows.
+    /// - `overrides` carries the `set <col> …` clause: per-column pins
+    ///   that take precedence over the heuristic generator (D2).
     Seed {
         table: String,
+        /// `Some(col)` → column-fill mode (UPDATE existing rows);
+        /// `None` → whole-row generation (INSERT new rows).
+        target_column: Option<String>,
         count: Option<u64>,
+        overrides: Vec<SeedOverride>,
         rng_seed: Option<u64>,
     },
     /// Replay a sequence of DSL commands from a file. Each line
@@ -647,6 +656,38 @@ impl RowFilter {
     }
 }
 
+/// One `set <col> …` override on a `seed` command (ADR-0048 D2, Phase 2).
+///
+/// The user can pin a column's generated values to a constant, a
+/// pick-list, an explicit named generator, or a range — overriding the
+/// per-column heuristic the executor would otherwise pick. `column` is
+/// the user-typed column name (validated against the table at execution,
+/// like every other column slot).
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct SeedOverride {
+    pub column: String,
+    pub kind: SeedOverrideKind,
+}
+
+/// The four `set` override forms (ADR-0048 D2).
+///
+/// Values arrive as the DSL's `Value` (quoted text / unquoted number —
+/// dates are quoted text per the D2 amendment); the `Generator` name is
+/// a raw string validated at execution because `src/dsl` cannot depend
+/// on `src/seed` (the curated vocabulary lives there).
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum SeedOverrideKind {
+    /// `set status = 'pending'` — every row gets the constant.
+    Fixed(Value),
+    /// `set role in ('admin', 'editor')` — uniform pick from the list.
+    PickList(Vec<Value>),
+    /// `set work_addr as email` — force the named generator (D9).
+    Generator(String),
+    /// `set price between 10 and 100` — uniform in `[low, high]`;
+    /// numeric or (quoted) date bounds per the destination column type.
+    Range { low: Value, high: Value },
+}
+
 /// A complex WHERE expression (ADR-0026 §4).
 ///
 /// Built by `grammar::expr::build_expr` from the flat