Add vector_concat() and vector_slice() functions

README-vector-functions.md

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+# Vector Concat and Slice Functions for libSQL
+
+This project adds two new vector functions to libSQL to maintain compatibility with Turso:
+
+1. `vector_concat(X, Y)` - Concatenates two vectors of the same type.
+2. `vector_slice(X, start_idx, end_idx)` - Extracts a subvector from start_idx (inclusive) to end_idx (exclusive).
+
+## Implementation
+
+The implementation is based on the existing vector functions in libSQL. The new functions are added to the `vector.c` file in the `libsql-sqlite3/src` directory.
+
+### `vector_concat(X, Y)`
+
+This function concatenates two vectors of the same type. It performs the following steps:
+
+1. Parse the two input vectors
+2. Check that both vectors are of the same type
+3. Allocate a new vector with dimensions equal to the sum of the dimensions of the input vectors
+4. Copy the data from both vectors into the new vector
+5. Return the new vector
+
+### `vector_slice(X, start_idx, end_idx)`
+
+This function extracts a slice of a vector from start_idx (inclusive) to end_idx (exclusive). It performs the following steps:
+
+1. Parse the input vector
+2. Validate the start and end indices:
+   - Both must be non-negative
+   - start_idx must not be greater than end_idx
+   - Both must be within the bounds of the vector
+3. Allocate a new vector with dimensions equal to end_idx - start_idx
+4. Copy the appropriate slice of data from the input vector to the new vector
+5. Return the new vector
+
+Note: FLOAT1BIT vectors are not yet supported for the slice operation due to the complexity of bit-by-bit extraction.
+
+## Usage
+
+```sql
+-- Create a test table with a vector column
+CREATE TABLE test_vectors (
+  id INTEGER PRIMARY KEY,
+  vec VECTOR
+);
+
+-- Insert some test vectors
+INSERT INTO test_vectors VALUES (1, vector32(1, 2, 3, 4, 5));
+INSERT INTO test_vectors VALUES (2, vector32(6, 7, 8, 9, 10));
+
+-- Concatenate vectors
+SELECT vector_extract(vector_concat(vec, vector32(11, 12, 13))) FROM test_vectors WHERE id = 1;
+-- Returns: [1.0, 2.0, 3.0, 4.0, 5.0, 11.0, 12.0, 13.0]
+
+-- Slice a vector
+SELECT vector_extract(vector_slice(vec, 1, 4)) FROM test_vectors WHERE id = 1;
+-- Returns: [2.0, 3.0, 4.0]
+```
+
+## Testing
+
+A test file `test_vector_functions.sql` is provided to verify the implementation.
+
+## Building
+
+The implementation is integrated directly into the libSQL SQLite fork. To build it, follow the standard libSQL build process.
+
+## References
+
+- Turso Implementation: https://github.com/tursodatabase/turso/pull/2336
+- libSQL Issue: https://github.com/tursodatabase/libsql/issues/2136