Complete vec0 DELETE: zero data, reclaim empty chunks, fix metadata rc bug

When a row is deleted from a vec0 virtual table, the rowid slot in
_chunks.rowids and vector data in _vector_chunksNN.vectors are now
zeroed out (previously left as stale data, tracked in #54). When all
rows in a chunk are deleted (validity bitmap all zeros), the chunk and
its associated vector/metadata shadow table rows are reclaimed.

- Add vec0Update_Delete_ClearRowid to zero the rowid blob slot
- Add vec0Update_Delete_ClearVectors to zero all vector blob slots
- Add vec0Update_Delete_DeleteChunkIfEmpty to detect and delete
  fully-empty chunks from _chunks, _vector_chunksNN, _metadatachunksNN
- Fix missing rc check in ClearMetadata loop (bug: errors were silently
  ignored)
- Fix vec0_new_chunk to explicitly set _rowid_ on shadow table INSERTs
  (SHADOW_TABLE_ROWID_QUIRK: "rowid PRIMARY KEY" without INTEGER type
  is not a true rowid alias, causing blob_open failures after chunk
  delete+recreate cycles)
- Add 13 new tests covering rowid/vector zeroing, chunk reclamation,
  metadata/auxiliary/partition/text-PK/int8/bit variants, and
  page_count shrinkage verification
- Add vec0-delete-completeness fuzz target
- Update snapshots for new delete zeroing behavior

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/fuzz/vec0-delete-completeness.c

@@ -0,0 +1,114 @@
+#include <stdint.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include "sqlite-vec.h"
+#include "sqlite3.h"
+#include <assert.h>
+
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
+  if (size < 6) return 0;
+
+  int rc;
+  sqlite3 *db;
+  sqlite3_stmt *stmtInsert = NULL;
+  sqlite3_stmt *stmtDelete = NULL;
+  sqlite3_stmt *stmtScan = NULL;
+  sqlite3_stmt *stmtCount = NULL;
+
+  rc = sqlite3_open(":memory:", &db);
+  assert(rc == SQLITE_OK);
+  rc = sqlite3_vec_init(db, NULL, NULL);
+  assert(rc == SQLITE_OK);
+
+  rc = sqlite3_exec(db,
+    "CREATE VIRTUAL TABLE v USING vec0(emb float[4], chunk_size=4)",
+    NULL, NULL, NULL);
+  if (rc != SQLITE_OK) { sqlite3_close(db); return 0; }
+
+  sqlite3_prepare_v2(db,
+    "INSERT INTO v(rowid, emb) VALUES (?, ?)", -1, &stmtInsert, NULL);
+  sqlite3_prepare_v2(db,
+    "DELETE FROM v WHERE rowid = ?", -1, &stmtDelete, NULL);
+  sqlite3_prepare_v2(db,
+    "SELECT rowid FROM v", -1, &stmtScan, NULL);
+
+  if (!stmtInsert || !stmtDelete || !stmtScan) goto cleanup;
+
+  size_t i = 0;
+  while (i + 2 <= size) {
+    uint8_t op = data[i++] % 3;
+    uint8_t rowid_byte = data[i++];
+    int64_t rowid = (int64_t)(rowid_byte % 16) + 1;
+
+    switch (op) {
+      case 0: {
+        // INSERT
+        float vec[4] = {0.0f, 0.0f, 0.0f, 0.0f};
+        for (int j = 0; j < 4 && i < size; j++, i++) {
+          vec[j] = (float)((int8_t)data[i]) / 10.0f;
+        }
+        sqlite3_reset(stmtInsert);
+        sqlite3_bind_int64(stmtInsert, 1, rowid);
+        sqlite3_bind_blob(stmtInsert, 2, vec, sizeof(vec), SQLITE_TRANSIENT);
+        sqlite3_step(stmtInsert);
+        break;
+      }
+      case 1: {
+        // DELETE
+        sqlite3_reset(stmtDelete);
+        sqlite3_bind_int64(stmtDelete, 1, rowid);
+        sqlite3_step(stmtDelete);
+        break;
+      }
+      case 2: {
+        // Full scan
+        sqlite3_reset(stmtScan);
+        while (sqlite3_step(stmtScan) == SQLITE_ROW) {}
+        break;
+      }
+    }
+  }
+
+  // Delete all remaining rows
+  sqlite3_exec(db, "DELETE FROM v", NULL, NULL, NULL);
+
+  // Assert all shadow tables are empty after full deletion
+  sqlite3_prepare_v2(db,
+    "SELECT count(*) FROM v_rowids", -1, &stmtCount, NULL);
+  if (stmtCount) {
+    rc = sqlite3_step(stmtCount);
+    assert(rc == SQLITE_ROW);
+    assert(sqlite3_column_int(stmtCount, 0) == 0);
+    sqlite3_finalize(stmtCount);
+    stmtCount = NULL;
+  }
+
+  sqlite3_prepare_v2(db,
+    "SELECT count(*) FROM v_chunks", -1, &stmtCount, NULL);
+  if (stmtCount) {
+    rc = sqlite3_step(stmtCount);
+    assert(rc == SQLITE_ROW);
+    assert(sqlite3_column_int(stmtCount, 0) == 0);
+    sqlite3_finalize(stmtCount);
+    stmtCount = NULL;
+  }
+
+  sqlite3_prepare_v2(db,
+    "SELECT count(*) FROM v_vector_chunks00", -1, &stmtCount, NULL);
+  if (stmtCount) {
+    rc = sqlite3_step(stmtCount);
+    assert(rc == SQLITE_ROW);
+    assert(sqlite3_column_int(stmtCount, 0) == 0);
+    sqlite3_finalize(stmtCount);
+    stmtCount = NULL;
+  }
+
+cleanup:
+  sqlite3_finalize(stmtInsert);
+  sqlite3_finalize(stmtDelete);
+  sqlite3_finalize(stmtScan);
+  sqlite3_close(db);
+  return 0;
+}