Update test-unit.c and unittest.rs functions to enforce pre-existing behavior

- Expand sqlite-vec-internal.h with scanner/tokenizer types, vector column
  definition types, and parser function declarations
- Fix min_idx declaration to match actual C signature (add candidates,
  bTaken, k_used params)
- Compile test-unit with -DSQLITE_CORE and link vendor/sqlite3.c so
  sqlite3 API functions (sqlite3_strnicmp, sqlite3_mprintf, etc.) resolve
- Add unit tests for vec0_token_next, Vec0Scanner, and
  vec0_parse_vector_column
- Fix Rust build.rs to define SQLITE_CORE and compile vendor/sqlite3.c
- Fix Rust min_idx FFI signature and wrapper to match actual C function

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

tests/build.rs

@@ -1,13 +1,12 @@
-use std::env;
-use std::path::{Path, PathBuf};
-use std::process::Command;
-
 fn main() {
     cc::Build::new()
         .file("../sqlite-vec.c")
-        .include(".")
+        .file("../vendor/sqlite3.c")
+        .define("SQLITE_CORE", None)
+        .include("../vendor")
+        .include("..")
         .static_flag(true)
         .compile("sqlite-vec-internal");
-    println!("cargo:rerun-if-changed=usleep.c");
     println!("cargo:rerun-if-changed=build.rs");
+    println!("cargo:rerun-if-changed=../sqlite-vec.c");
 }