Merge 'Direct schema mutation – add instruction' from Levy A.

<img width="960" height="205" alt="image" src="https://github.com/user-
attachments/assets/f60a2133-dfe4-4411-9a7c-7283eb073875" />
<img width="944" height="504" alt="image" src="https://github.com/user-
attachments/assets/9383c8e2-4d8d-40b9-8ace-825ca3cf8682" />
```
`ALTER TABLE _ ADD COLUMN _`/limbo_add_column/
                        time:   [2.1199 ms 2.1921 ms 2.2756 ms]
                        change: [-85.983% -85.416% -84.716%] (p = 0.00 < 0.05)
                        Performance has improved.
Found 13 outliers among 100 measurements (13.00%)
  6 (6.00%) high mild
  7 (7.00%) high severe
`ALTER TABLE _ ADD COLUMN _`/sqlite_add_column/
                        time:   [10.358 ms 10.404 ms 10.469 ms]
                        change: [-6.2566% -2.3515% +0.2046%] (p = 0.21 > 0.05)
                        No change in performance detected.
Found 14 outliers among 100 measurements (14.00%)
  2 (2.00%) high mild
  12 (12.00%) high severe
  ```

Reviewed-by: Preston Thorpe <preston@turso.tech>

Closes #2482

core/translate/alter.rs

@@ -181,7 +181,7 @@ pub fn translate_alter_table(
                 }
             }
 
-            btree.columns.push(column);
+            btree.columns.push(column.clone());
 
             let sql = btree.to_sql();
             let mut escaped = String::with_capacity(sql.len());
@@ -219,9 +219,9 @@ pub fn translate_alter_table(
                         value: schema.schema_version as i32 + 1,
                         p5: 0,
                     });
-                    program.emit_insn(Insn::ParseSchema {
-                        db: usize::MAX, // TODO: This value is unused, change when we do something with it
-                        where_clause: None,
+                    program.emit_insn(Insn::AddColumn {
+                        table: table_name.to_owned(),
+                        column,
                     });
                 },
             )?

core/vdbe/execute.rs

@@ -6871,13 +6871,13 @@ pub fn op_drop_column(
     pager: &Rc<Pager>,
     mv_store: Option<&Arc<MvStore>>,
 ) -> Result<InsnFunctionStepResult> {
-    let Insn::DropColumn {
-        table,
-        column_index,
-    } = insn
-    else {
-        unreachable!("unexpected Insn {:?}", insn)
-    };
+    load_insn!(
+        DropColumn {
+            table,
+            column_index
+        },
+        insn
+    );
 
     let conn = program.connection.clone();
 
@@ -6887,16 +6887,45 @@ pub fn op_drop_column(
             .get_mut(table)
             .expect("table being renamed should be in schema");
 
-        {
-            let table = Arc::make_mut(table);
+        let table = Arc::make_mut(table);
 
-            let Table::BTree(btree) = table else {
-                panic!("only btree tables can be renamed");
-            };
+        let Table::BTree(btree) = table else {
+            panic!("only btree tables can be renamed");
+        };
 
-            let btree = Arc::make_mut(btree);
-            btree.columns.remove(*column_index)
-        }
+        let btree = Arc::make_mut(btree);
+        btree.columns.remove(*column_index)
+    });
+
+    state.pc += 1;
+    Ok(InsnFunctionStepResult::Step)
+}
+
+pub fn op_add_column(
+    program: &Program,
+    state: &mut ProgramState,
+    insn: &Insn,
+    pager: &Rc<Pager>,
+    mv_store: Option<&Arc<MvStore>>,
+) -> Result<InsnFunctionStepResult> {
+    load_insn!(AddColumn { table, column }, insn);
+
+    let conn = program.connection.clone();
+
+    conn.with_schema_mut(|schema| {
+        let table = schema
+            .tables
+            .get_mut(table)
+            .expect("table being renamed should be in schema");
+
+        let table = Arc::make_mut(table);
+
+        let Table::BTree(btree) = table else {
+            panic!("only btree tables can be renamed");
+        };
+
+        let btree = Arc::make_mut(btree);
+        btree.columns.push(column.clone())
     });
 
     state.pc += 1;

core/vdbe/explain.rs

@@ -1645,6 +1645,15 @@ pub fn insn_to_str(
                 0,
                 format!("drop_column({table}, {column_index})"),
             ),
+            Insn::AddColumn { table, column } => (
+                "AddColumn",
+                0,
+                0,
+                0,
+                Value::build_text(""),
+                0,
+                format!("add_column({table}, {column:?})"),
+            ),
             Insn::MaxPgcnt { db, dest, new_max } => (
                 "MaxPgcnt",
                 *db as i32,

core/vdbe/insn.rs

@@ -5,7 +5,7 @@ use std::{
 
 use super::{execute, AggFunc, BranchOffset, CursorID, FuncCtx, InsnFunction, PageIdx};
 use crate::{
-    schema::{Affinity, BTreeTable, Index},
+    schema::{Affinity, BTreeTable, Column, Index},
     storage::{pager::CreateBTreeFlags, wal::CheckpointMode},
     translate::collate::CollationSeq,
     Value,
@@ -1034,6 +1034,10 @@ pub enum Insn {
         table: String,
         column_index: usize,
     },
+    AddColumn {
+        table: String,
+        column: Column,
+    },
     /// Try to set the maximum page count for database P1 to the value in P3.
     /// Do not let the maximum page count fall below the current page count and
     /// do not change the maximum page count value if P3==0.
@@ -1182,6 +1186,7 @@ impl Insn {
             Insn::IntegrityCk { .. } => execute::op_integrity_check,
             Insn::RenameTable { .. } => execute::op_rename_table,
             Insn::DropColumn { .. } => execute::op_drop_column,
+            Insn::AddColumn { .. } => execute::op_add_column,
             Insn::MaxPgcnt { .. } => execute::op_max_pgcnt,
             Insn::JournalMode { .. } => execute::op_journal_mode,
         }