feat(scenegraph): add hash codec for bits/tensor/bytes/cam_row conversion

Introduce hash_codec module providing bidirectional encoding/decoding:
- bits_tensor_to_hash_bytes / hash_bytes_to_bits_array
- bits_tensor_to_cam_row
- hash_bytes_to_cam_row / cam_row_to_hash_bytes

mini-nav/scenegraph/hash_codec.py

@@ -0,0 +1,62 @@
+from __future__ import annotations
+
+import numpy as np
+import torch
+
+DEFAULT_HASH_WIDTH = 512
+
+
+def _validate_width(width: int) -> None:
+    if width <= 0:
+        raise ValueError("width must be greater than 0")
+    if width % 8 != 0:
+        raise ValueError("width must be divisible by 8")
+
+
+def _expected_byte_length(width: int) -> int:
+    _validate_width(width)
+    return width // 8
+
+
+def bits_tensor_to_hash_bytes(bits: torch.Tensor, *, width: int = 512) -> bytes:
+    _validate_width(width)
+    flat = bits.detach().cpu().flatten()
+    if flat.numel() != width:
+        raise ValueError(
+            f"Input tensor must have exactly {width} values, got {flat.numel()}"
+        )
+    bit_array = (flat.numpy() > 0).astype(np.uint8)
+    return np.packbits(bit_array).tobytes()
+
+
+def hash_bytes_to_bits_array(hash_bytes: bytes, *, width: int = 512) -> np.ndarray:
+    expected = _expected_byte_length(width)
+    if len(hash_bytes) != expected:
+        raise ValueError(
+            f"hash_bytes must be exactly {expected} bytes, got {len(hash_bytes)}"
+        )
+    byte_array = np.frombuffer(hash_bytes, dtype=np.uint8)
+    return np.unpackbits(byte_array)[:width].astype(np.uint8, copy=False)
+
+
+def hash_bytes_to_cam_row(hash_bytes: bytes, *, width: int = 512) -> int:
+    expected = _expected_byte_length(width)
+    if len(hash_bytes) != expected:
+        raise ValueError(
+            f"hash_bytes must be exactly {expected} bytes, got {len(hash_bytes)}"
+        )
+    return int.from_bytes(hash_bytes, byteorder="big", signed=False)
+
+
+def cam_row_to_hash_bytes(cam_row: int, *, width: int = 512) -> bytes:
+    if not (0 <= cam_row < 1 << width):
+        raise ValueError(
+            f"cam_row {cam_row} is out of range [0, 2**{width})"
+        )
+    expected = _expected_byte_length(width)
+    return int(cam_row).to_bytes(expected, byteorder="big", signed=False)
+
+
+def bits_tensor_to_cam_row(bits: torch.Tensor, *, width: int = 512) -> int:
+    hash_bytes = bits_tensor_to_hash_bytes(bits, width=width)
+    return hash_bytes_to_cam_row(hash_bytes, width=width)