Add comprehensive fuzz testing infrastructure with 6 new targets

- Fix numpy.c: tautology bug (|| → &&), infinite loop, and missing
  sqlite3_vec_numpy_init call
- Replace tests/fuzz/Makefile: auto-detect clang, add UBSAN, macOS
  ld_classic workaround, generic build rules for all 10 targets
- Add 6 new fuzz targets: shadow-corrupt (corrupted shadow tables),
  vec0-operations (INSERT/DELETE/query sequences), scalar-functions
  (all 18 SQL scalar functions), vec0-create-full (CREATE + lifecycle),
  metadata-columns (metadata/auxiliary columns), vec-each (vec_each TVF)
- Add seed corpora for shadow-corrupt, vec0-operations, exec, and json
- Add fuzz-build/fuzz-quick/fuzz-long targets to root Makefile

All 10 targets verified building and running on macOS ARM (Apple Silicon).

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

tests/fuzz/vec0-create-full.c

@@ -0,0 +1,43 @@
+#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) {
+  int rc;
+  sqlite3 *db;
+
+  rc = sqlite3_open(":memory:", &db);
+  assert(rc == SQLITE_OK);
+  rc = sqlite3_vec_init(db, NULL, NULL);
+  assert(rc == SQLITE_OK);
+
+  // Create table with fuzz input as column definitions
+  sqlite3_str *s = sqlite3_str_new(NULL);
+  assert(s);
+  sqlite3_str_appendall(s, "CREATE VIRTUAL TABLE v USING vec0(");
+  sqlite3_str_appendf(s, "%.*s", (int)size, data);
+  sqlite3_str_appendall(s, ")");
+  char *zSql = sqlite3_str_finish(s);
+  assert(zSql);
+
+  rc = sqlite3_exec(db, zSql, NULL, NULL, NULL);
+  sqlite3_free(zSql);
+
+  if (rc == SQLITE_OK) {
+    // Table was created — try to use it. These may fail (errors are fine),
+    // but must never crash.
+    sqlite3_exec(db, "INSERT INTO v(rowid) VALUES (1)", NULL, NULL, NULL);
+    sqlite3_exec(db, "SELECT * FROM v", NULL, NULL, NULL);
+    sqlite3_exec(db, "SELECT * FROM v WHERE rowid = 1", NULL, NULL, NULL);
+    sqlite3_exec(db, "DELETE FROM v WHERE rowid = 1", NULL, NULL, NULL);
+    sqlite3_exec(db, "DROP TABLE v", NULL, NULL, NULL);
+  }
+
+  sqlite3_close(db);
+  return 0;
+}