test: add numerical accuracy integration tests for bitnet-quantization (#838)

## Summary

Add six new integration tests to
`crates/bitnet-quantization/tests/integration_tests.rs` that validate
actual numerical correctness of the quantization implementations.

## New Tests

| Test | What it checks |
|------|---------------|
| `test_i2s_dequantize_known_values` | Hand-crafted I2_S block (byte
`0xE4`, scale 1.0) decodes to `[-2, -1, 0, 1]` |
| `test_tl1_lut_entry_count` | TL1 2-bit config has 4 LUT levels (2²),
packed bytes = `n × 2 / 8`, all codes ∈ `[0, 3]` |
| `test_tl2_dequantize_symmetry` | Codes `[0,1,2,3]` with scale 1.0
decode to symmetric `[-2,-1,0,1]` via `(code − shift) × scale` |
| `test_quantize_dequantize_round_trip_accuracy` | Seeded-random 64
values in `[-0.1, 0.1]`; I2_S reconstruction error < 0.1 per element |
| `test_qk256_block_size` | `QK256_BLOCK == 256` and `QK256_PACKED_BYTES
== 64`; validates `I2SQk256NoScale::new` |
| `test_zero_vector_quantizes_to_zero` | All-zeros input → all-zeros
output for I2S, TL1, and TL2 |

## Structural Change


## Test Run

```
cargo test -p bitnet-quantization --no-default-features --features cpu --test integration_tests

running 6 tests
test test_qk256_block_size ... ok
test test_i2s_dequantize_known_values ... ok
test test_tl2_dequantize_symmetry ... ok
test test_tl1_lut_entry_count ... ok
test test_quantize_dequantize_round_trip_accuracy ... ok
test test_zero_vector_quantizes_to_zero ... ok

test result: ok. 6 passed; 0 failed; 0 ignored
```

Full suite: zero failures across all bitnet-quantization test files.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: EffortlessSteven