ADR-0018 implementation: auto-fill contracts for serial and shortid

Generalises serial and shortid beyond their previous restricted
forms:

- `serial` is no longer restricted to single-column PK. Non-PK
  serial columns get an emitted UNIQUE constraint and use
  application-side MAX(col)+1 at INSERT time (rowid alias still
  drives the PK case for free; per ADR-0010 worker-thread
  serialisation, the read-then-insert sequence is safe).
- `shortid` columns auto-fill existing null cells when the
  column is materialised — `add column T: x (shortid)` on a
  non-empty table no longer leaves rows in a not-really-valid
  NULL state.
- `int -> serial` joins the type-change matrix as always-clean
  identity (closes the asymmetry vs `text -> shortid`); other
  sources are refused with a route-via-int hint.
- `change column T: x (serial|shortid)` fills null source
  cells with sequence / generated values in the same rebuild
  transaction.

Internal infrastructure:

- ReadColumn gains `unique: bool`; read_schema detects single-
  column UNIQUE indexes via pragma_index_list /
  pragma_index_info; schema_to_ddl emits inline UNIQUE for
  non-PK columns.
- ColumnSchema (persistence) gains `unique: bool` so the flag
  survives YAML round-trip and rebuild-from-text reconstructs
  it faithfully — preserves the "serial -> int leaves UNIQUE
  in place" promise across save/load cycles.
- ChangeColumnTypeResult.client_side now carries `auto_filled`
  + `auto_fill_kind` alongside `transformed` + `lossy`; the
  app handler renders separate note lines when both apply.
- AddColumnResult is a new return type carrying pre-rendered
  [client-side] note lines for the auto-fill paths.

Tests: 519 -> 534 (+15). Clippy clean.

src/persistence/mod.rs

@@ -100,6 +100,13 @@ pub struct TableSchema {
 pub struct ColumnSchema {
     pub name: String,
     pub user_type: Type,
+    /// Whether this column carries a single-column UNIQUE
+    /// constraint (ADR-0018 §4). Stored explicitly in the
+    /// project YAML so that a `serial → int` round-trip
+    /// (which leaves UNIQUE in place) is preserved across a
+    /// save/load cycle. Defaults to `false` when missing in
+    /// older project files.
+    pub unique: bool,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq)]
@@ -318,6 +325,7 @@ mod tests {
             columns: vec![ColumnSchema {
                 name: "Name".to_string(),
                 user_type: Type::Text,
+                unique: false,
             }],
             rows: vec![vec![CellValue::Text("Alice".to_string())]],
         };