ADR-0017 implementation: per-cell type-change with override flags

Replaces the placeholder "trust STRICT" body of do_change_column_type
with the per-cell transformer matrix from ADR-0017. Adds:

- src/type_change.rs: CellOutcome { Clean / Lossy / Incompatible }
  + transform_cell + static_refusal covering every matrix pair
  from §3 (54 unit tests).
- --force-conversion and --dont-convert flags on `change column`
  (mutually exclusive at parse time per §5).
- Refined PK rule (§4.1): refused only when the column has an
  inbound FK and fk_target_type would change. Outbound-FK columns
  still refused outright (§4.2). PK / shortid uniqueness checked
  post-transformation (§4.3).
- Bordered diagnostic tables (lossy / incompatible / collision)
  via the pretty-table renderer (§7) — uses ADR-0016's primitives.
- [client-side] success note (§6) when any cell was rewritten.
- Friendly wrapper for engine-level errors under --dont-convert
  so no engine vocabulary leaks (ADR-0002 user-facing posture).

ADR-0017 §3 + §7 amended in place (with user sign-off): serial->int
added explicitly to the always-clean matrix, and diagnostic rows
identify themselves by PK value(s) rather than positional indices
(SQLite returns rows unordered without ORDER BY, so positional
"row 5" is unaddressable).

Tests: 449 -> 517 (+68). Clippy clean with nursery lints.

src/runtime.rs

@@ -29,7 +29,8 @@ use crate::action::Action;
 use crate::app::App;
 use crate::cli::Args;
 use crate::db::{
-    DataResult, Database, DbError, DeleteResult, InsertResult, TableDescription, UpdateResult,
+    ChangeColumnTypeResult, DataResult, Database, DbError, DeleteResult, InsertResult,
+    TableDescription, UpdateResult,
 };
 use crate::dsl::Command;
 use crate::event::AppEvent;
@@ -946,6 +947,10 @@ fn spawn_dsl_dispatch(
                 command: command.clone(),
                 result,
             },
+            Ok(CommandOutcome::ChangeColumn(result)) => AppEvent::DslChangeColumnSucceeded {
+                command: command.clone(),
+                result,
+            },
             Err(DbError::PersistenceFatal {
                 operation,
                 path,
@@ -982,6 +987,7 @@ enum CommandOutcome {
     Insert(InsertResult),
     Update(UpdateResult),
     Delete(DeleteResult),
+    ChangeColumn(ChangeColumnTypeResult),
 }
 
 /// Execute a parsed user command and return either a typed
@@ -1019,10 +1025,15 @@ async fn execute_command_typed(
             .rename_column(table, old, new, src)
             .await
             .map(|d| CommandOutcome::Schema(Some(d))),
-        Command::ChangeColumnType { table, column, ty } => database
-            .change_column_type(table, column, ty, src)
+        Command::ChangeColumnType {
+            table,
+            column,
+            ty,
+            mode,
+        } => database
+            .change_column_type(table, column, ty, mode, src)
             .await
-            .map(|d| CommandOutcome::Schema(Some(d))),
+            .map(CommandOutcome::ChangeColumn),
         Command::AddRelationship {
             name,
             parent_table,